Remove GPS Time Tracking from the Orbit Class

[kylekrol]

Currently, the orb::Orbit class tracks GPS time in nanoseconds internally and performs some validity checks on it's value. For our purposes, always tracking GPS time with an orbit may cause issues given potentially "discontinuities" in time that may be caused by Teensy clock drift. Currently, the orbit estimator doesn't even make use of the tracked GPS time.

We should pull tracked GPS time out of the general orbit class and then fix up the downstream implementations accordingly.