Big mesh refactoring

Main changes:

- mesh pointer added to MeshEntity, all topology accessors redirected
  through it to the mesh
- storage of points and subentity orientations moved from MeshEntity to
  Mesh
- removed MeshEntityIndex - MeshEntity always stores its index as the
  GlobalIndexType
- removed entity storage from Mesh - mesh entities can be generated on
  the fly

Plus some minor related simplifications, mainly in the mesh initializer.