Introduce `hasGeometry` capability

At the DUNE Developer Meeting 2013 it was decided that grids must provide subentities of all codimensions, however they don't have to provide a geometry for all of them. It was further decided to implement a hasGeometry capability to indicate for which entites a geometry is provided.

I assume this means adding

template <typename Entity>
struct hasGeometry
{
  const static bool v = false;
};

in the Dune::Capabilities namespace?

Or should this be a property of the grid? i.e. template<typename Grid, int codim> struct hasGeometry?

Reference: https://www.dune-project.org/community/meetings/2013-09-devmeeting/#subentity-of-all-co-dimensions