Refactor l2 injection

[schlafly]

This refactors the L2 source injection code in order to give it a cleaner API.

[codecov]

Codecov Report

Attention: Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.95%. Comparing base (d4af8fd) to head (981b906).
Report is 46 commits behind head on main.

Files with missing lines	Patch %	Lines
romanisim/image.py	95.65%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
- Coverage   89.95%   89.95%   -0.01%     
==========================================
  Files          17       17              
  Lines        1792     1831      +39     
==========================================
+ Hits         1612     1647      +35     
- Misses        180      184       +4     

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[schlafly]

@zacharyburnett , sorry to bug you again, but building the wheels is failing due to failure to find a compiler? Have you seen that before? https://github.com/spacetelescope/romanisim/actions/runs/10322214023/job/28576941035?pr=138#step:7:1699