Moves application of transformation from micro to reference element from visitor to PDELab interface

We previously completely avoided loopy for temporary declaration.
We now rely on it again, but have DuneTemporaryVariable, which is
capable of doing a bit more things (and at the same time handles
the indexing question)

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.