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.

Note: needs ufl patch

as a consequence introduce template parameters

UFL did not like  my way of replacing some args with 1 as it
destroyed the shape information. It now remains in the expression,
but triggers code generation separately.