Orbitals continued

[attipaci]

• Tweak enum novas_orbital_system to o contain a proper reference coordinate system instead of just an equator type.
• Renamed novas_set_orbital_pole() to novas_set_orbsys_pole(), which now takes a coordinate reference system as an input, in line with the above.
• Exposed existing gcrs_to_tod() and tod_to_gcrs() functions.
• Added gcrs_to_mod() and mod_to_gcrs() functions for convenience.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.70%. Comparing base (2cb5a59) to head (e85a28b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #98   +/-   ##
=======================================
  Coverage   99.70%   99.70%           
=======================================
  Files           8        8           
  Lines        3024     3050   +26     
  Branches      575      575           
=======================================
+ Hits         3015     3041   +26     
  Partials        9        9           

Flag	Coverage Δ
unittests	99.70% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/novas.c	99.62% <100.00%> (+<0.01%)	⬆️
src/super.c	100.00% <100.00%> (ø)

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