This patch modifies the hash used for the name of the function produced by
evaluable.compile, in order to reuse it as nutils hash.

This patch adds a simplification for LoopSum, that promotes inflations for
which the dofmaps are independent of index.

This patch removes the background colour of graphviz nodes that are not visited
once. For functions that are called more then once, this helps to distinguish
nodes with cached values from those that evaluate in a short time span.

This patch adds the argument_degree method to evaluable arrays, which returns
the highest power in a specific argument if the evaluable is polynomial in that
argument; or raises a NotPolynomial error otherwise.

This patch adds function.factor and evaluable.factor for the creation of
equivalent sparse polynomials. See the documentation of evaluable.factor for
details.

This patch replaces the trialshapes dictionary in System, which contains only
the shapes of the trial arguments, by the argshapes dictionary, which contains
all of the evaluable's arguments. The reason for the change is that the broader
set of arguments can be used for all sorts of argument checks prior to
initiating a solve.

This patch renames the System.step parameters timetarget to timearg and
historysuffix to suffix for brevity.