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.

Sum factorization!

This is the new merge requests that supersedes both !42 and !45.

Also bumps submodules in various places, as loopy has had a rough week.

Also fixes #47, #20 .

See merge request !47

With some upstreamed fixes relevant to us.

Needs a small fix in loopy (MR opened).

It is a memory reinterpretation

... instead of reimplementing it all over the place

It otherwise is a pain to debug problems arising in cgen.

Merge branch 'feature/sumfactorization' of ssh://parcomp-git.iwr.uni-heidelberg.de:20022/dominic/dune-perftool into feature/sumfactorization

Conflicts:
	python/dune/perftool/sumfact/amatrix.py

Conflicts:
	python/dune/perftool/pdelab/quadrature.py
	python/dune/perftool/sumfact/amatrix.py

Get endless temporaries who circularly switch around their base storage.
Any temporary may have different shape or dim_tags, reinterpreting the
data at will.

Feature/eliminate dimension variable

Fixes #42

See merge request !46

Feature/update loopy

Some more upstream changes.

Test suite passes.

See merge request !44

Feature/fix stokes stress

This fixes #44.

See merge request !43

Implement test function splitting

As a step towards sum factorization, this implements a splitting of test function modified arguments from the rest of the form.

While at it, I introduced a data structure `ModifiedArgument`, which uses `pytools.Record` and is much better for debugging.

See merge request !41