[Dune] Dune::IdSet

[Guo Hua]

Hi everyone

I feel confused when reading the explanations of these two functions
in the class Dune::Grid< dim, dimworld, ct, GridFamily >.

const Codim< 0 >::GlobalIdSet & globalIdSet () const
 	return const reference to the grids global id set

const Codim< 0 >::LocalIdSet & localIdSet () const
 	return const reference to the grids local id set 

Would you mind to make it more clear? Especially the differences of two.
Thanks a lot!

Cheers,
Hua