Update test_model to version that compares tempo/tempo2.
Revert Time->mpmath convert in spindown
Added ability to set the JPL ephemeris when loading SPICE files
Use new TOA reading convenience function.  Other minor changes.
Added default for planet Shapiro in get_TOAs
Added info about how to compile ERFA from ERFA-fetch
Added param specifying which ephem was used for each TOA
Put script in code block