Make embarrassingly parallelizable builds build in parallel

[jondequinor]

Make embarrassingly parallelizable builds build in parallel. This means that for each level in the dependency graph, all nodes at this level should be built in parallel.

This means that if the graph looks like this:

                  python
                    / \
                   /   \
                  /     \
                 /       \
           setuptools   wheel
               / \        /\
              /   \      /  \
             /     \    /    \
            /       \  /      \
           /         \/        \
     contextlib2  cachetools  enum34

three batches of build will occur:

1. python
2. setuptools and wheel
3. contextlib2, cachetools and then enum34

etc.

If, for any reason, some build is not parallelizable, it should be explicitly marked as such in repository.yml.

pyscal:
  v0.5.1:
    source: git://github.com/equinor/pyscal
    fetch: git
    make: sh
    makefile: src/setup-py.sh
    maintainer: havb
    depends:
      - numpy
      - pandas
      - python
      - …
    build_isolation: yes

So, if a package is for some reason not possible to build in parallel with other packages, it can get build_isolation—which currently means that it is built not in parallel with other packages.