getting rid of hard-coded MPI_COMM_WORLD

[Axel]

hi!

this is a RFC to the cp2k developers that hang out here.
i'd like to change the code, so that cp2k can be run from
a "supervisor" MPI program. that would mean to get rid of
the (few!) hardcoded MPI_COMM_WORLD constants in
message_passing.F and f77_interface.F so that one can
launch a cp2k calculation on a group communicator (which
of course would default to MPI_COMM_WORLD).

any comments, suggestions, caveats would be highly appreciated.

thanks,
   axel.