Gitlab ci runner setup

ok, I've attached a little runner (an old laptop of mine ;)), and written a gitlab-ci script that runs the tests and also checks for 'black' code-style, all seems to be passing right now (after another small fix): https://gitlab.aei.uni-hannover.de/GregAshton/PyFstat/pipelines

/cc @dkeitel

mentioned in merge request !21 (merged)

merged

mentioned in commit 6057a8ed

Thanks for fixing this @ReinhardPrix ! :)

yw ;-) btw, given that this is literally just an old laptop standing around in my office, I can't guarantee very good support or reliability/uptime necessarily, so this should only be considered a 'best effort' fix for the moment. Longer term, if this is something useful or gets used more, it might indeed be safer to move to a service like gitlab.com or github with free CI support provided. Anyway, I think for now it's perfectly fine.