Deprecation warnings from Dune::Hybrid::size

At some points, e.g. in the multidomain assembler, we use for Dune::MultiTypeBlockMatrix/Dune::MultiTypeBlockVector m:

using namespace Dune::Hybrid;
forEach(m, [](auto& row){
   ...
});

This accesses Dune::MultiTypeBlockMatrix::size which is deprecated since before dune 2.7 and causes the warning. Unfortunately this means we need to use a bit more verbose syntax:

using namespace Dune::Hybrid;
forEach(integralRange(M::N()), [](auto& i){
   auto& row = m[i];
   ...
});

should work.

A proposal to allow ::N() to de deduced from a range passed to Dune::Hybrid::forEach (https://gitlab.dune-project.org/core/dune-common/-/merge_requests/803) has been rejected for good reasons. The might be something like rows(m) in the future... but that doesn't help now.