Avoid having generation magic use global context

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 is a followup MR for !63

See merge request !64

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.

Multiple kernel infrastructure

Also: constructor is a loopy kernel.
This fixes #40.

See merge request !63

Feature/vectorize poisson

See merge request !61

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

Add vectorization support to dune-perftool

See merge request !58

Do not use continue stuff. Instead precompute the quantities.