// Entity functionality.

For inter-part boundary entities, the APIs look sufficient but require
some tuning And there is one function for convenience which we may want 
to consider. See comments below:

1) prefix_addCopyEnt will require remote_entity_handle along with 
remote_part_id (entity_handle is local on-process handle).
prefix_removeCopyEnt may have either remote_part_id
or remote_entity_handle (or both).

2) prefix_getEntityOwner should return owner_part_id and owner_copy_handle
(can be remote_entity_handle). Also need to provide prefix_getEntityOwnerPart
and prefix_getEntityOnwerCopy. prefix_getOwnedEnt is then a repeat.

3) For convenience, we may provide prefix_getResidencePartIds, see example
in the end.

4) Some entity functions take both partition-handle and part-handle 
whereas some take only partition-handle, it is not clear which ones and why.
For example, prefix_isEntOwner take both hanldes whereas prefix_getEntityOwner
takes only partition-handle.

5) In functiona names, we may want to use Ent for Entity and rmv for remove.
For example, prefix_getEntOwner and prefix_rmvCopyEnt.

Example (as pseudo C code): uses residence parts functionality.
All owner vertices at inter-part boundary wants to update some field value
on its remote copies. In this example, the amount of sends from any part to 
every other part is determined.
(assume a part per process for this case).

-------------------------------------------------------------------
// nsends defines comm. volume from current part to every other part
int *nsends = calloc(num_parts,sizeof(int));

while(pBdryVtx= getNext()) { // iterate over vertices on inter-part boundary

  if(prefix_isEntOwner()) { // check if this copy is owner

    int numCopies = getNumCopies(); // get num. copies of this owner vertex
    int *resParts = getResidencePartIds(); // get residence part IDs
    for(int iCopy=0; iCopy<numCopies; iCopy++)
      // increment for part with ID of resParts[iCopy]
      nsends[resParts[iCopy]]++; 
  }

}

// do communication of nsends from each part to other part to find nrecvs,
// then each part knows how much it has to receive from every other part.

// allocate values_send and values_receive and do communication of field values. 

free(nsends);

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