Automatically detect & exclude periods with extreme motion rates

[barentsen]

This is a follow-up of #2.

Because tess-ephem downloads and interpolates ephemeris from JPL Horizons with a pre-determined resolution (currently 12 hours), it will not yield accurate results for extremely fast-moving objects.

The good news:

1. We can automatically detect periods affected by extreme motion from the ephemeris data.
2. It would likely be fine to automatically exclude those periods from the ephemeris analysis, because TESS cannot usefully observe objects moving at many degrees per day.

Hence, I propose we address this issue by automatically excluding episodes of extreme motion from the TESS visibility assessment. The maximum rate of motion supported could be documented in the documentation, and a warning message could be issued when such extreme motion is encountered for an object.