It is not necessary anymore

It is applied to the function call...

Omit cmake generated main function

Closes #118

See merge request dominic/dune-perftool!235

As a step towards making the driver the main function.

Implement possibility of functions that provide their own visitor

See merge request dominic/dune-perftool!234

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.

[cmake] Fix dependency tracking for multiple operators

Closes #108

See merge request dominic/dune-perftool!233

Implement matrix free block preconditioner operators

Closes #114

See merge request dominic/dune-perftool!232

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.

cleanup facet jump confusion

Closes #103 and #105

See merge request dominic/dune-perftool!229

New vectorization features

See merge request dominic/dune-perftool!227

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