Funnily, we were only able to calculate symmetric problems so far.

Need to use handler from pymbolic as loopy codegen handlers
are too involved for standalone usage.

That was a bad idea from the beginning! It lead to a situation,
where calls from within a generator inherited their context tags.
Should be fixed now.

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)