Add offset to spectral transformations to enable shifted slits

[oczoske]

This is a bit of a hack to enable simulations of offset slits in MICADO. The offset is in the dispersion direction and observations in the nominal and offset slits are meant to fill in missing data due to the gaps in the detector array. A proper implementation would use separate trace files computed for both slit positions. Here, offsets are determined from the CRVALiD in the FOV header and added to the x and y arrays from which the transformations are computed. The CRVALiD values are zero for nominal slits and non-zero for shifted slits. This works for MICADO, but it needs to be tested that it doesn't break METIS and other spectrographs.

Related to AstarVienna/irdb#308

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.85%. Comparing base (327b28c) to head (95aaa7a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #875      +/-   ##
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+ Coverage   74.83%   74.85%   +0.02%     
==========================================
  Files          69       69              
  Lines        8868     8876       +8     
==========================================
+ Hits         6636     6644       +8     
  Misses       2232     2232              

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