ENH: generalize MPI broadcast to basic and user-defined MPI types

- simplify and rationalize some of the broadcast methods for more code
  reuse.

  The bottom level UPstream::broadcast is now always to/from "root=0".
  This was previously passed as a default parameter, but never used
  anything other than '0' in the code. Fixing it as '0' makes it
  consistent with the 'top-down' logical for node-based broadcast.

applications/test/globalIndex3/Test-globalIndex3.cxx

@@ -345,8 +345,8 @@ static void reportOffsets(const globalIndex& gi)
 
         UPstream::broadcast
         (
-            allOffsets.data_bytes(),
-            allOffsets.size_bytes(),
+            allOffsets.data(),
+            allOffsets.size(),
             interNodeComm
         );
     }