Fixes #1186 (closed)
Introduces a helper that tells us whether the grid supports multithreaded iteration over the grid view. There is a capability in Dune to assert that property at compile time. But this hasn't been specialized yet for the current release (will come in Dune 2.9). We use the capability as fall-back but then provide specializations for the most common grid managers. They are all expected to be thread-safe already in 2.8 (which is refelected by the updated specialization of the traits in upcoming Dune 2.9). However, dune-uggrid still has a problem for MPI-parallel runs (https://gitlab.dune-project.org/core/dune-grid/-/merge_requests/597) which is why we disable multithreading in this case.