This implements a utility function to determine the geometrical extents of a
set of points.

So far this really only covers insertion.

Entry impls Positionable, but any user-defined type may do so as well.

Many items in the tree module are mere implementation details and do not need
to be exposed publicly.

A separate structure can be constructed from the tree to associate arbitrary
data from a closure to each node. On this structure one can then execute
computational queries.

Also it is no longer unsafe as this is not quite the right use case for that
keyword.

This slightly changes the semantics, but no test was looking into this
peculiarity so it's okay to remove it.

There are two traits: Tree defines tree construction alone, whereas TreeWalk
wraps up computational queries on the trees.

This is arguably the more useful quantity than the extent (i.e. the
half-width).

The construction routines of Node have been integrated into NodeWithData to
form a new type NTree. The construction benchmarks have been adapted. The query
functionality does not work yet.

All tests have been reinstated and run successfully.

Tree interface

The operational behaviour of a tree is defined by what was TreeWalk before.
Hence, TreeWalk has been renamed to Tree. The former Tree trait was removed on
the other hand, as the constructors it defines do not hold generally. One may
for example write a tree without associated data (such as the current Node) and
still implement Tree<P, N, O, ()> for it.

Every tree should at least provide information about its structure and
geometry, so that one can write generic algorithms that directly work with the
structure outside the tree implementation.

This separates these concerns and leads to a cleaner interface. For convenience
and clear semantics two new trivial wrapper traits `Tree` and `PureTree` have
been added to comprise the functionality that makes up trees with and without
associated data.

This also allows to benchmark `PureNTree` versus `NTree`.

Common tree abstractions now live in the `tree` module, the
dimension-unspecific implementations in `ntree` and `util` now also contains
`Positionable` and `Entry` as they fit nowhere else (although they might
warrant another module at some point.

It is not necessary to deal with the accumulation of values in the query
implementations. It is simpler to pass some callback that handles state changes
by closing over its environment.

Restructure traits

Update documentation

Updates the requirements on [nalgebra](https://github.com/rustsim/nalgebra) to permit the latest version.
- [Release notes](https://github.com/rustsim/nalgebra/releases)
- [Changelog](https://github.com/dimforge/nalgebra/blob/dev/CHANGELOG.md)
- [Commits](dimforge/nalgebra@v0.21.0...v0.22.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

…a-0.22

Update nalgebra requirement from 0.21 to 0.22

Updates the requirements on [approx](https://github.com/brendanzab/approx) to permit the latest version.
- [Release notes](https://github.com/brendanzab/approx/releases)
- [Commits](https://github.com/brendanzab/approx/commits)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Updates the requirements on [nalgebra](https://github.com/rustsim/nalgebra) to permit the latest version.
- [Release notes](https://github.com/rustsim/nalgebra/releases)
- [Changelog](https://github.com/dimforge/nalgebra/blob/dev/CHANGELOG.md)
- [Commits](dimforge/nalgebra@v0.22.0...v0.23.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Update approx requirement from 0.3 to 0.4

…a-0.23

Update nalgebra requirement from 0.22 to 0.23

Updates the requirements on [rand](https://github.com/rust-random/rand) to permit the latest version.
- [Release notes](https://github.com/rust-random/rand/releases)
- [Changelog](https://github.com/rust-random/rand/blob/master/CHANGELOG.md)
- [Commits](rust-random/rand@0.7.0...0.8.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Update rand requirement from 0.7 to 0.8

Updates the requirements on [itertools](https://github.com/bluss/rust-itertools) to permit the latest version.
- [Release notes](https://github.com/bluss/rust-itertools/releases)
- [Changelog](https://github.com/rust-itertools/itertools/blob/master/CHANGELOG.md)
- [Commits](rust-itertools/itertools@v0.9.0...v0.10.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Updates the requirements on [nalgebra](https://github.com/rustsim/nalgebra) to permit the latest version.
- [Release notes](https://github.com/rustsim/nalgebra/releases)
- [Changelog](https://github.com/dimforge/nalgebra/blob/dev/CHANGELOG.md)
- [Commits](dimforge/nalgebra@v0.23.0...v0.24.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

…a-0.24

Update nalgebra requirement from 0.23 to 0.24

Updates the requirements on [quickcheck](https://github.com/BurntSushi/quickcheck) to permit the latest version.
- [Release notes](https://github.com/BurntSushi/quickcheck/releases)
- [Commits](BurntSushi/quickcheck@quickcheck_macros-0.9.0...1.0.1)

Signed-off-by: dependabot-preview[bot] <[email protected]>

…eck-1.0

Update quickcheck requirement from 0.9 to 1.0

Updates the requirements on [nalgebra](https://github.com/dimforge/nalgebra) to permit the latest version.
- [Release notes](https://github.com/dimforge/nalgebra/releases)
- [Changelog](https://github.com/dimforge/nalgebra/blob/dev/CHANGELOG.md)
- [Commits](dimforge/nalgebra@v0.24.0...v0.25.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

…a-0.25

Update nalgebra requirement from 0.24 to 0.25

Updates the requirements on [approx](https://github.com/brendanzab/approx) to permit the latest version.
- [Release notes](https://github.com/brendanzab/approx/releases)
- [Commits](brendanzab/approx@v0.4.0...v0.5.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Update approx requirement from 0.4 to 0.5

…-file

Upgrade to GitHub-native Dependabot

…ls-0.10

Update itertools requirement from 0.9 to 0.10

Updates the requirements on [nalgebra](https://github.com/dimforge/nalgebra) to permit the latest version.
- [Release notes](https://github.com/dimforge/nalgebra/releases)
- [Changelog](https://github.com/dimforge/nalgebra/blob/dev/CHANGELOG.md)
- [Commits](dimforge/nalgebra@v0.25.0...v0.28.0)

---
updated-dependencies:
- dependency-name: nalgebra
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

…a-0.28

Update nalgebra requirement from 0.25 to 0.28

Updates the requirements on [nalgebra](https://github.com/dimforge/nalgebra) to permit the latest version.
- [Release notes](https://github.com/dimforge/nalgebra/releases)
- [Changelog](https://github.com/dimforge/nalgebra/blob/dev/CHANGELOG.md)
- [Commits](dimforge/nalgebra@v0.28.0...v0.29.0)

---
updated-dependencies:
- dependency-name: nalgebra
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

…a-0.29

Update nalgebra requirement from 0.28 to 0.29