Support for running on Swift simulation output

[jchelly]

I'd like to be able to run HBT on output from Swift (https://github.com/SWIFTSIM/swiftsim). Initially in postprocessing, but maybe on the fly eventually. @Kambrian - do you know if anyone has anyone already done this? If not I could have a go at implementing it. Swift can already write out FoF information and the snapshots are similar to EAGLE/APOSTLE so I think it should not be too hard to make HBT run on Swift output.

[Kambrian]

Not as far as I know. Please go ahead! @jchelly

[jchelly]

Thanks Jiaxin. I think I have it working on small snapshots but to run efficiently on larger simulations I'll probably need to use parallel HDF5 because Swift writes all the particles to one giant snapshot file.

I also spotted a minor bug that causes an error message on shutdown with Intel MPI. See #12 for my attempt at a fix.