[md][staggered] Fix localAssembler for clang

* due to a bug in clang (https://stackoverflow.com/a/47849501),
  myMultiTypeBlockvector.size() (which is static constexpr) has to be
  changed to MultiTypeBlockVector::size() to be used as a constexpr
  template argument
* probably a similar issue with using Dune::Hybrid::forEach and std::get<>