Deprecate the method Entity<0>::count<codim>()

As decided in

http://users.dune-project.org/doodles/14

and

http://users.dune-project.org/doodles/15

this method is being replaced with the new Entity<0>::subEntities(unsigned int).
Use new method Entity<0>::subEntities instead of deprecated Entity<0>::count
Use new method Entity<0>::subEntities instead of deprecated method Entity<0>::count
Test the old Entity<0>::count method only if DISABLE_DEPRECATED_METHOD_CHECK is not set