This will be needed to replace nonlinearities in Richards code
with a cubic spline evaluation without bloating dune-perftool with
the implementation details of such procedure.

Also provides a test, that defines a custom square function.

Instationary stuff not fixed so far.

Do not annotate FiniteElements anymore, further split driver code

and split mini files to avoid reuse of old vtu files!

based on dune-python!