Refactor: remove comm_2D from Parallel_2D (Useful Information: BLACS encapsulates MPI and maintains its own internal data structure for MPI communicators)

[jinzx10]

BLACS encapsulates MPI and maintains its own internal data structure for MPI communicators, and it is inadvisable to have MPI and BLACS functions intertwined. Given a BLACS context, one can always obtain its underlying MPI communicator via blacs_get followed by blacs2sys_handle. So, in theory, in the presence of a BLACS context, there's no need to hold its underlying MPI communicator explicitly. Maintaining both the BLACS context and its underlying MPI communicator within the same class not only cause confusion, but also make it bug-prone if not synchronized appropriately. This is particularly troublesome given that they are both public members and widely used in the code.

What's changed?

• comm_2D, a former member of class Parallel_2D (hence also a member of Parallel_Orbitals), is removed from the code. Users who really need the MPI communicator upon which the BLACS grid is built may call the comm() function to obtain the underlying communicator associated with blacs_ctxt held by the class.
• a few BLACS utility functions for broadcasting data is added to the header "blacs_connector.h" and a generic interface is provided.