Working:
- Driver block
- Solving and visualization

Not working:
- Error calculation

These methods are part of the driver block class interface and should also be
generated if they are not used from the main function.

[ ] Go through system part -> GFS
[ ] Export boundary grid function

IT is both simplicial and quadrilateral...

So far, we were a bit sloppy about which basis we choose from the UFL
counterpart. This commit implements the necessary 1-to-1 mapping.

This reverts commit f83a804e

We previously used bool() on the constraints expression to find
out whether we need to use the DIrichletConstraintsAssembler.
However, UFL does the stupid thing of evaulating

bool(eq(x[0], 0))

to False, because x[0] and 0 are obvioulsy not the same. Veeeery
bad behaviour for a symbolic language.

Note: This is still structured for now

Instationary stuff not fixed so far.

Much more telling name...

These might sound completely silly, but they are useful when writing
code generic w.r.t. the number of components

otherwise my paraview throws me segfault.

To comply with latest pdelab.

- Use TensorProductElement in one example.

- We still use the sumfact option to swicth to the sum factorization
  code branch. This way it is still possible to easily swicth between
  sumfact and non sumfact code.

- Only TensorProductElements with the same degree in all directions
  will work. Anisotropie and adaption of quadrature rule will happen
  in the next commits.