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

---

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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