Dso overhaul

A new backend for storing and loading deep sky (and potentially other) objects.

Details in project.org.

This constitutes the first part of the work I am doing for GSOC 2021.

Blockers:

• CI (docker image has outdated libs)
• shift to C++14, connected to the above
• knewstuff
• mechanism for shipping openngc by default
  • has to be improved though
• a little polish on the csv import
• gpl banners
• documentation?
• compression of catalog files