Rename the new method Entity<0>::count(uint codim) to Entity<0>::subEntities(uint codim)

This has been discussed in FS 1455 (https://dune-project.org/flyspray/index.php?do=details&task_id=1455)

This patch by itself is not an interface change, because the method
in question is not part of the interface yet.
Test the new Entity<0>::subEntities method for codim==dim