So far, we had the opcounter cast to Dune::FieldVector
of size 1. Now it defines the correct type.

Simple test passes already

Avoid explicit doubles.

Avoid explicit doubles.

No substantial changes there.

They are not used in a senseful way anyway and loopy
stumbled over them.

This is currently only used for level 4 instrumentation,
as these start and stop markers were messing heavily with
dependencies in the sum factorization code. This way, the necessary but
ugly work of adding instrumentation is separated much more
nicely.