Deprecation stuff and things

closes #53

closes #13

This reverts commit e54351e2.

It would be better if we could move classes only with
deprecation warning in place for a while.

Dailywork timkeil

[ci] adds testing on arch docker

 remove last dune-fem compat code

revert_threadstorage_update

This reverts commit 061f0659, as we
decided against this in dune-xt-grid (see the imaginary protocol in our
heads).

Dependencies updates

since xt-la depends on xt-common and dune-istl, we can just depend on
istl here too

[ThreadResultPropagator] rewire for shared_ptrs, simplify for 1 copy …

Some threading updates

the internal lazy creation of objects was actually a hindrance to
how we use this class

dailywork-ftschindler

tests relying on more than one worker should now
explicitly set 'threading.max_count' (via ini file)

[cmake/py] make all tests execute in dune's venv