Initialize MPI regardless of process type.

Added a mechanism to initialize MPI on all processes, even client processes to
simplify development of readers/plugins that use MPI.

The client processes (paraview, pvpython) now take a command line arguments
--mpi, --no-mpi that can be used to request enabling/disabling of calling
MPI_Init() on those processes during startup.

A CMake flag, PARAVIEW_INITIALIZE_MPI_ON_CLIENT can be used to control th
default, used when the new command line arguments are not used.
PARAVIEW_INITIALIZE_MPI_ON_CLIENT is OFF by default. When set to ON, the
ParaView client executables will init MPI when no arguments are specified.
They command line argumnets (--mpi/--no-mpi) can still override the default and
hence provide a mechanism at runtime to change this.

Change-Id: Iff90ed3d65358b6e940f8d8be59f98d0ca861684
Merge topic 'always-initialize-mpi' into next

4574019 Initialize MPI regardless of process type.