Support printing in other time systems thank just UTC and TAI

[ChristopherRabotin]

This should be relatively straight forward since the Gregorian computation uses the seconds representation of a given epoch in a given time system.

Idea:

• {} for UTC (implemented)
• {:x} for TAI (implemented)
• {:X} for TT
• {:e} for TDB (the E is seen as a shortcut to ephemeris, and those typically use TDB)
• {:E} for ET
• {:p} for UNIX (as a pointer to computer systems in a way)
• {:o} for GPS, where the O represents an orbit