Improve accuracy and speed of planet functions

[taldcroft]

Description

This includes a few improvements / fixes:

• Factor out the key method in jplephem.spk that actually computes planet positions, then wrap this in numba for about 25 times speedup (for that method only).
• Use a specialized version of convert_time_format for time format conversions that is much faster. It sacrifices about a msec of accuracy which is fine here.
• Fix a bug where the input JD time for SPK was in the UTC scale instead of the required TDB scale. This ~69 sec difference reduced the accuracy of planet positions.
• Fix an issue in get_planet_chandra where the orbit ephemeris values were being interpolated to the nearest neighbor instead of linearly. Given the 5 minute sampling this makes a big difference.

Overall the get_planet_barycentric function is about 10-12 times faster for scalar or small-array valued inputs.

The accuracy of Venus planet predictions was improved from about 3-4 arcsec to under 0.2 arcsec relative to JPL Horizons.

Interface impacts

Planet positions will change slightly for the better.

Testing

Unit tests

• Mac

(ska3-perf) ➜  chandra_aca git:(fast-scalar-planet-barycentric) git rev-parse HEAD                
bc34a28fc9a57272e2e745ec5bca25da69910504
(ska3-perf) ➜  chandra_aca git:(fast-scalar-planet-barycentric) pytest chandra_aca
================================================== test session starts ===================================================
platform darwin -- Python 3.10.8, pytest-7.2.1, pluggy-1.0.0
rootdir: /Users/aldcroft/git, configfile: pytest.ini
plugins: timeout-2.1.0, anyio-3.6.2
collected 197 items                                                                                                      

chandra_aca/tests/test_aca_image.py ..............                                                                 [  7%]
chandra_aca/tests/test_all.py ........................                                                             [ 19%]
chandra_aca/tests/test_attitude.py .............................................................                   [ 50%]
chandra_aca/tests/test_dark_model.py ....                                                                          [ 52%]
chandra_aca/tests/test_drift.py ..........................                                                         [ 65%]
chandra_aca/tests/test_maude_decom.py ...............                                                              [ 73%]
chandra_aca/tests/test_planets.py .............                                                                    [ 79%]
chandra_aca/tests/test_psf.py ...                                                                                  [ 81%]
chandra_aca/tests/test_residuals.py ss...                                                                          [ 83%]
chandra_aca/tests/test_star_probs.py ................................                                              [100%]

============================================ 195 passed, 2 skipped in 31.86s =============================================

Independent check of unit tests by Javier

• OSX:

Functional tests

See the two new validation notebooks.

[javierggt]

I went over the changes and they make sense to me. I verified the test outputs and the validation notebooks.

It looks good to me.