Remove global functions and data associated with a default COMM_WORLD

This removes:

libMesh::COMM_WORLD
libMesh::Parallel::n_processors()
libMesh::Parallel::processor_id()

when the default COMM is disabled.  This is extremely helpful when trying to track down places in application code that need to be updated.

I also added a different version of all of these called

libMesh::GLOBAL_COMM_WORLD
libMesh::Parallel::global_n_processors()
libMesh::Parallel::global_processor_id()

That always uses the communicator libMesh was initialized with.  This is handy for when you truly do need "global" MPI information.