requires an upstream fix

This removes:
- The now obsolete quadrature_loop_statement.
- Quadrature weights are now a globalarg and we do not create a
temporary anymore.

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)

In sum factorization we will have different temporary shape and access.

* Enable backend functions which are *not* generators
* Use the sumfac switch as a default switch

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.

Needs a small fix in loopy (MR opened).

It is a memory reinterpretation