Add niriss mocking

[karllark]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 9.09091% with 20 lines in your changes missing coverage. Please review.

Project coverage is 43.18%. Comparing base (819b545) to head (80fb6c9).
Report is 24 commits behind head on master.

Files with missing lines	Patch %	Lines
...easure_extinction/utils/make_obsdata_from_model.py	5.55%	17 Missing ⚠️
measure_extinction/merge_obsspec.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   41.04%   43.18%   +2.14%     
==========================================
  Files          21       22       +1     
  Lines        2863     3830     +967     
==========================================
+ Hits         1175     1654     +479     
- Misses       1688     2176     +488     

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