Deprecate Dune::array

It seems that std::array can now be assumed to exist so that Dune::array is no longer necessary? This seems consistent with e.g. pdelab/dune-pdelab!62 (merged). At the same time, Dune::array is currently not even deprecated as far as I can tell and still used in dune/geometry/genericgeometry/referenceelements.hh.

If nobody objects, I'd like to go ahead, deprecate the class and replace the (single) remaining use of Dune::array in the core modules (through std::array).