Swift velociraptor

Allows VELOCIraptor to be called from within SWIFT as a library call. Use -x to run structure finding. Need to provide config file for VELOCIraptor and VELOCIraptor units in the SWIFT parameter file, examples of both can be found in SmallCosmoVolume.

Building SWIFT and VELOCIraptor together is outlined in: https://gitlab.cosma.dur.ac.uk/swift/swiftsim/wikis/build%20swift%20velociraptor%20on%20COSMA