Separated the command line arguments and actual interesting stuff out of
main() to simplify things a bit.

This is probably not necessary for normal usage, but might be useful
when running via Slurm or similar.

This is mainly so that I can work on this via CLion.

Code was originally in:
https://git.ecdf.ed.ac.uk/dmckain/parallel-scratchpad/tree/master/c-xthi