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

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.

Changed default formats for VTKWriter and VTUWriter to binary and compressed, compressed falls back to binary if zlib is not available

The method was unused (and does not make much sense for the format
anyway).

Fixes #6

Updates for CUDA 11

See merge request !71