Proposed changes

I realized I didn't add backwards compatibility support in #5957 for if the Horizons.h5 file didn't exist. This PR now allows you to specify the path to the Horizons.h5 file. If the file doesn't exist, the user is told to run postprocess-id on the ID to generate this file.

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments