general/globals.[ch].
Renamed class WrappedOStream to OutStream and use a different
implementation in order to support the case when the OutStream is passed
as a parameter of type std::ostream.
In class MPI_Session, added an MPI_Comm parameter to both constructors,
with a default value of MPI_COMM_WORLD.
Hide the global variable MFEM_COMM_WORLD and allow access (read and
write) with global functions: GetGlobalMPI_Comm() and
SetGlobalMPI_Comm() - in the future, we may want to perform some tasks
when changing the "global" MPI communicator.
In mfem_error(), use GetGlobalMPI_Comm() instead of MPI_COMM_WORLD.
Minor formatting changes and #include reordering.
Veselin Dobrev