JOSS: Emulator docstring and warning

[odunbar]

Purpose

Addressing #294

Content

• Added a comprehensive dosctring for Emulator(...)
• Added a warning when not providing obs_noise_cov as a keyword

Due to the way manifests/GH actions work, will need a later PR to add the docs for this change

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• I have read and checked the items on the review checklist.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.27%. Comparing base (acf1051) to head (a9d285d).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #299      +/-   ##
==========================================
+ Coverage   88.17%   88.27%   +0.09%     
==========================================
  Files           8        8              
  Lines        1184     1185       +1     
==========================================
+ Hits         1044     1046       +2     
+ Misses        140      139       -1     

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