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)

Needs a small fix in loopy (MR opened).

It is a memory reinterpretation

... instead of reimplementing it all over the place

including CallInstructions for accumulation etc.

major remodelling of the DuneTarget

as a consequence introduce template parameters

laplace is again proceeding to code generation

much more clever!