Extract direct concepts from requires expression

In concept checking experiments it seems that it is better for shorter error messages to extract concept requirements from the requires expression. That way, the compiler can use short-circuit and stops evaluating concepts if one the first requirement is not fulfilled. This makes error messages more readable. Also, in the error message, the arguments to the compound requires expression are not shown if the failing concept is outside.

I have removed the EntitySet concept in favor of a more elaborate GridView concept. The idea of an entity set comes from dune-pdelab and dune-functions, maybe others, but it not established in dune-grid. Here we use the concept GridView and this very often. Thus this concept should be provided directly, without a long chain of sub-concepts with non-common names.