Improve --loci argument options

[mstubb]

The --loci command line argument takes a space-separated list of locus names. This means that it must be followed by another command line option so that argparse knows when to stop looking for locus names. This isn't ideal and should be changed to something less annoying.

[canergen]

I recognized an additional problem on macOS (I am not sure whether it's OS specific), where it needs two spaces between the different loci (--loci A B G D works whereas --loci A B G D shows errors parsing the rest of the command).

[mstubb]

Thanks. We'll look into this when we get around to improving the general behaviour of this option