The backend selecting mechanism needs all modules to be imported.
This commit restructures the package to be able to do that. Placement
of some functions felt weird, but this can still be changed.
No more placements in __init__.py anymore though, as this will
result in cyclic dependencies.

and an example with stokes stress

They are concepts that are not really necessary with local
storage, that I do not really want to explain to other people.

This reverts commit d0a87348.

major remodelling of the DuneTarget

This involves adding a wrapper around the LocalBasisCache, that
has value semantics (at the cost of only allowing indexing with []
into the result and not perform any other operation on it).

Some work on parameter classes, a bugfix on the splitting algorithm
some simplifications in the generation stage