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

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.79%. Comparing base (9a4120b) to head (19904ed).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
scopesim/effects/spectral_trace_list_utils.py	40.00%	3 Missing ⚠️
scopesim/effects/spectral_trace_list.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #875      +/-   ##
==========================================
+ Coverage   74.76%   74.79%   +0.03%     
==========================================
  Files          69       69              
  Lines        8936     8944       +8     
==========================================
+ Hits         6681     6690       +9     
+ Misses       2255     2254       -1     

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[teutoburg]

Seems useful 👍