Refactoring distributed mesh and grid synchronizers: using DistributedMesh as the primary template parameter

TNLReader does not fit, but it will be probably removed anyway...

Moved Mesh_impl.h, MeshEntity_impl.h and Traverser_impl.h from MeshDetails to Meshes and changed suffix to .hpp

Consequently, we always store the numbers of all entities, even those
that don't have any subentity or superentity mappings enabled.