Including code to process a subset of inputs, fixing unit tests

[astronomerritt]

Fixes #982.

• There is now an optional command-line option, --process-subset, which allows you to process just a subset of your input orbit/parameters files. By default this is 1/1 (i.e., process everything).
• Usage/example: --process subset 2/10 will process the second tenth of the objects in the orbit file.
• I integrated Mario's code for this and also fixed it so it works with external ephemeris files.
• Wrote unit tests.

Fixes #995.

• Implemented Drew's fix for the two unit tests which fail when sorcha-addons is installed.

Review Checklist for Source Code Changes

• Does pip install still work?
• Have you written a unit test for any new functions?
• Do all the units tests run successfully?
• Does Sorcha run successfully on a test set of input files/databases?
• Have you used black on the files you have updated to confirm python programming style guide enforcement?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.08%. Comparing base (9aae130) to head (954e785).
Report is 39 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #998      +/-   ##
==========================================
+ Coverage   80.89%   81.08%   +0.18%     
==========================================
  Files          69       69              
  Lines        3094     3124      +30     
==========================================
+ Hits         2503     2533      +30     
  Misses        591      591              

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[mschwamb]

@astronomerritt can you resolve conflicts?

[astronomerritt]

Done here too!

[mschwamb]

Some issues #1009

[mschwamb]

There's an issue ticket submitted about the performance of this PR

[mschwamb]

At this point, this PR is too far out of date, that I think we should come back to this at a later time with a fresh PR.