New catalogue concerning the deep sky objects

The program now displays objects from the OpenNGC catalog (https://github.com/mattiaverga/OpenNGC) whose positions and data are much more reliable than those contained in the old SAC catalog.

This catalogue was built by merging information from NASA/IPAC Extragalactic Database, HyperLEDA, Simbad and HEASARC and includes about 15,000 objects.

More information on its creation can be found here: https://github.com/mattiaverga/OpenNGC/blob/master/README.md