Make anisotropic quadrature order possible for non DG examples.

- 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.

The y ar enot properly implemented right now, do not seem
to be necessary and conflict with recent changes in loopy.

The way we currently iterate over children of mxied elements
was plain wrong for nested mixed elements. The tests that
revealed this will soon follow.

So far, we used a global context to tell the normal visitor,
that it is generating for driver code. This was feasible, as
we only had one differing implementation. With more differences,
it is better style to derive from the visitor and implement
the differences there.

The indexing information needs to be stored and reset
for visiting of the second branch.