Add the ability to niche_ellipse() to handle three isotopes

[benjaminhlina]

This is still a WIP and is not ready to merged at this moment

[codecov]

Codecov Report

❌ Patch coverage is 38.95349% with 210 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.24%. Comparing base (ee6306d) to head (5728eaa).
⚠️ Report is 49 commits behind head on main.

Files with missing lines	Patch %	Lines
R/niche_ellipse.R	38.06%	179 Missing ⚠️
R/create_isotope_pairs.R	0.00%	31 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #18       +/-   ##
===========================================
- Coverage   99.57%   76.24%   -23.34%     
===========================================
  Files          10       11        +1     
  Lines         702      926      +224     
===========================================
+ Hits          699      706        +7     
- Misses          3      220      +217     

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