Implements unit tests for orbit conversion utils, fixes obliquity of …

[bernardinelli]

…the ecliptic value. Also fixes one character in the lightcurve demo notebook

Fixes #680 .

Describe your changes.

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 ✅

Comparison is base (b9b5cdf) 74.34% compared to head (77c0d26) 74.54%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #748      +/-   ##
==========================================
+ Coverage   74.34%   74.54%   +0.19%     
==========================================
  Files          66       66              
  Lines        3025     3025              
==========================================
+ Hits         2249     2255       +6     
+ Misses        776      770       -6     

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