Correct UT in calculation of TDB; closes #1938
Also update TDB, TCB in test done in documentation
Merge pull request #1939 from mhvk/time-correct-ut-in-tdb

Correct UT in calculation of TDB; closes #1938
Introduce a simple EarthLocation class, subclassed from CartesianPoints
Includes erfa gc2gd so that one can transform XYZ to geodetic
Use location exclusively, deprecating lon and lat (and removing height)
Move conversion of u, v, to "km" to inside the call to erfa