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

• [x] 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
- [x] OSX:

### Functional tests
<!-- Describe and document results of any functional tests, otherwise leave the text below -->
See the two new validation notebooks.