Bugfix grid transmission

[samaloney]

The grid orientation angle do not have to be "flipped" by 180 deg when we compute the offset angle of the flare location relative to the slats (in arcseconds). The grid orientation angles are indeed defined as in Figure 2 of The STIX Imaging Concept paper (Massa et al., 2023).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.56%. Comparing base (d22e0f1) to head (7deb9d7).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   73.50%   73.56%   +0.05%     
==========================================
  Files          32       32              
  Lines        2012     2016       +4     
==========================================
+ Hits         1479     1483       +4     
  Misses        533      533              

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