Constraints parameter classes are basically just grid functions returning
bools. Going through interpolate.py reduces lots of reduntant code.

Working:
- Driver block
- Solving and visualization

Not working:
- Error calculation

It makes the code generation workflow more complicated without
contributing anything substantial

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

Do not annotate FiniteElements anymore, further split driver code

though still mixed up by name with dirichlet bc.

Do not annotate FiniteElements anymore, further split driver code