fix fp mask emission

[oczoske]

This fixes the flux from the opaque parts of the focal-plane mask. We had used erroneously a high reflectivity for the mask, whereas the conservative case should use a high emissivity. In fact, using Eq.(17) of RvB's document, the intensity from the mask is independent of the emissivity and is just pure black-body intensity.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.85%. Comparing base (8eefa0e) to head (ab91b47).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #583      +/-   ##
==========================================
- Coverage   76.85%   76.85%   -0.01%     
==========================================
  Files          69       69              
  Lines        8572     8571       -1     
==========================================
- Hits         6588     6587       -1     
  Misses       1984     1984              

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