Validation on TimeOfArrival and Observation MJD values #6

Currently, there is no validation on values that are supposed to represent MJD datetimes. This causes problems if values are too far outside of the range that AstroPy knows how to deal with when, e.g., applying a barycentric correction.

Suggestion: Add an extra clean() function to any model with such an MJD field that checks if the given values are within some allowed, sensible range. Might need to investigate a programmatic way to choose said range (e.g. try/except for doing an actual barycentric correction?)