I downloaded the airless_Moon_dt.txt file from JPL Horizons so that
I could reverse engineer their DeltaT function. But keeping it in
the horizons directory caused unit test breakage. Moved to delta_t
directory so dtplot can still use it to make graphs.
Verified that the JPL checker works for apparent coordinates,
equatorial-of-date and horizontal, when I tell the JPL Horizons
tool to use its "airless" (no refraction) model.
Still need to implement refraction for at least horizontal coordinates
(where it really matters for rise/set times).
Used the JPL Horizons web interface at
https://ssd.jpl.nasa.gov/horizons.cgi
to generate test data for astrometric RA/DEC,
apparent RA/DEC, and apparent az/alt.
Will use this as a secondary unit test to confirm
that I'm not fooling myself by incorrectly using the
NOVAS C 3.1 code for unit tests.
