ligo-skymap-from-samples --instruments option can eat samples input argument
The nargs='+' in the instruments option in the argument parser of ligo-skymap-from-samples will eat all subsequent strings until another --option is found. If the --instruments option is passed last before the input samples file, it will consume the samples file. This causes the code to think there has been no input file given. The use of the old --samples option gets around this but it's a potential pitfall for users.