utils: add funcs for converting Time to mpmath
phase: minor docstring edit.
spindown: use mpmath for phase calc, return Phase object.
Merge branch 'scott'; fix conflicting utils.py

Conflicts:
	utils.py
Add list of external packages to README.
Added PosVel class and am computing SSB and telecope posns and vels now
Fixed J2000 epoch def, improved ET precision
Fixed precession/nutation so that we have ~1 m observatory precision
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.
astrometry: return barcentric freq without Units (for now)