Use new logic for gathering data in MeshFreeInterpolation

I didn't attempt to ascertain what was actually incorrect about the previous MPI logic in this function... but it was definitely wrong.  You can see examples of it here: https://github.com/idaholab/moose/issues/3743

Rather than diagnose the actual problem I decided just to replace all of the MPI shenanigans with much simpler code :smile: