It is not necessary anymore

It is applied to the function call...

As a step towards making the driver the main function.

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.

CMakes add_custom_command occasionally (undeterministically)
trips over when multiple files are defined as output. The
result is unnecessary regeneration upon every change of the
accompanying C++ source...

The commit fixes the issue by defining one custom command per
operator. This has the additional pro of generating more verbose
messages during generation. Implementation wise, CMake needed
to extract more information from the ini file, so I switched
to using dune-testtools Python-CMake interface.

It would be hard to implement on top of the latest feature and contradict
features projected for the near future.

fixes a nasty bug, where face information was omitted

By implementing repr such that instance addresses are
not part of sorting anymore