Specialize std::tuple_size for MultiTypeBlock*

... because we also specialize std::tuple_element and it derives from std::tuple after all.

mentioned in merge request dune-common!1209 (merged)

Notice that this makes Hybrid::size() work again for MultiTypeBlockMatrix, which was broken as a side-effect of removing it's size() method.

assigned to @oliver.sander

Cool. Thanks a lot.

merged

mentioned in commit d65d7acc