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.

It is a memory reinterpretation

... instead of reimplementing it all over the place

Very wild, totally WIP.

Note: needs ufl patch

Various fixes, including diagonal jacobian transformations.