This reverts commit 33672520

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.

I also made the order of the terms more consistent and it is now much
easier to compare the skeletons and the boundaries.

Instationary stuff not fixed so far.

There is way more reasons to interpolate a function into the
solution vector and we cover them with one code path:
* Dirichlet boundary conditions (strong ones)
* Initial conditions for instationary expressions
* Initial guesses for Newton solvers

To comply with latest pdelab.

It is badly implemented as is and does not offer value over
writing symbolic functions.

It is badly implemented as is and does not offer value over
writing symbolic functions.

Do not annotate FiniteElements anymore, further split driver code

Blockstructured tests may fail, since the interpolation of the exact solution is not suitable