astrometry: use MJDParameter for POSEPOCH
timing_model: parse D exponents in uncertainties.
astrometry: remove debug print statement.
timing_model: use "is" with None tests.
phase: wrap frac part to -0.5->0.5
astrometry: fix up Units in calculations.
astrometry: use correct position vector
astrometry: add radio freq doppler calculation
Add simple dispersion delay model component.
timing_model: rename delay/phase funcs.
Add simple model test prog.