[SCION] SCIONLab package for SIG

Hi there, we've created a binary package for the SCION IP Gateway (SIG) for SCIONLab. At the same time, we've updated the guide on how to install (which is now trivial) and configure the SIG; please have a look, at https://docs.scionlab.org/content/apps/remote_sig.html. We hope this makes the SIG a bit more approachable. We're looking forward to hear about the experiments that you set up with the SIG on SCIONLab. Cheers, Matthias