provide links to the issue OPM tracker in the readme. Patch by Roland Kaufmann
fix deallocation bug

we have to make sure that the grid and the time manager is still
allocated when the destructor of the problem is called. this fixes an
issue with test_adaptive2p2c. (and any other problem that makes calls
to the grid in its destructor. I'm pretty sure calling the grid in the
problem destructor is bad practice, but it should definitely not lead
to crashes.) Problem was found using address-sanitizer.
re-enable testing the tetrahedron sub-control volume geometries

It seems like the bug in CLang which made the binary fail was fixed.
grid communication handles: pass the codim of the communicated entities as parameter

In contrast to my previous believes, these handles only communicated
vertex data. the additional template argument is only required for the
contains() method, though. This is because there is no compatible
method in the Dune API for entity mappers, so we need to pass the
codim explicitly. annoying!