Make elements array an explicit type, i.e., fix to unsigned int as in the default interface

Summary

By making elements a dependent type on the member function templates, it is not possible to pass brace-enclosed initializer lists to the argument, as it is possible in the Dune::StructuredGridFactory default interface. This MR fixes this issue, by explicitly stating std::array<unsigned int, dim> for the input.

Discussion

I assume, that the introduction of int_t template parameters was due to the inconsistency of the StructuredGridFactory to YaspGrid (the first using unsigned int, the latter uses int as elements type). This is a different issue and unfortunately not so easy to solve. The introduces int_t changes the behavior of the function. So, this MR changes it back to the default.