Allow custom satellite systems in GNSS
Rinex files are limited to existing navigation constellations and within each system satellite numbers are limited to use two digits only.
Both limitations prevent performing simulations and generating Rinex observations files for prospective large constellations.
As SatelliteSystem
is an enumerate, users cannot extend it.
The enumerate should provision extra entries for user-defined extensions using available single letter keys (only 'C', 'E', 'G', 'I', 'J', 'R' and 'S' are used for existing systems today and 'M' is reserved for Mixed Rinex files). Users would remain responsible to split a constellation having 100 satellites or more into several key letters.
This feature is a follow-on to issue #1048 (closed).