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

We avoid the bug by not applying the transformation and casting the
result of jacobianInverseTransposed to a FieldMatrix. This way we do
not exploit the diagonal structure of the Transformation. Should get
fixed in the near future.

until poisson_neumann passes

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.