Fix bugs in return of group IDs to Swift (#94)
Fix bugs in return of group IDs to Swift (#94)

This affects the code in InvokeVelociraptorHydro which returns
the group info to swift.

 * Particles to be exported were being sorted to the start of the
   array instead of the end (as needed by MPISwiftExchange)

 * The array size passed to qsort when sorting the array was wrong

The velociraptor group IDs array output by Swift is still wrong,
however.
pelahi/VELOCIraptor-STF,pelahi/VELOCIraptor-STF,pelahi/VELOCIraptor-STF