deprecate/remove obsolete Pstream gather/scatter routines

The old Pstream::scatter routines (which were largely a misnomer) have been superseded by various broadcast routines, but were left in the code with #ifndef/#ifdef Foam_Pstream_scatter_nobroadcast guards.

Now noisily deprecate them, and remove the old manual tree communication in favour of MPI broadcast and/or serialize/de-serialize with wrapped Pstream::broadcast