Simple test passes already

Avoid explicit doubles.

They are not used in a senseful way anyway and loopy
stumbled over them.

This is currently only used for level 4 instrumentation,
as these start and stop markers were messing heavily with
dependencies in the sum factorization code. This way, the necessary but
ugly work of adding instrumentation is separated much more
nicely.

I was tired of the amount of ifs that changed behaviour
depending on stage 1 or 3. This is a cleaner approach.

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.