Merge branch 'feature/fancy-temporary-declaration' into 'master'

Reimplement temporary declaration

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)

This is finished and tests are passing, I will remove the WIP flag once a necessary upstream change is in master.

See merge request !52