Grating angle fix

[csbrasnett]

Remove general assumption that pattern is perfectly horizontal/vertical across the detector.

[codecov]

Codecov Report

❌ Patch coverage is 85.81560% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.74%. Comparing base (81d1f63) to head (de378cd).

Files with missing lines	Patch %	Lines
src/gratingcalibration/detector_calibration.py	89.79%	10 Missing ⚠️
src/gratingcalibration/beam_center_optimiser.py	82.14%	5 Missing ⚠️
src/gratingcalibration/main.py	20.00%	4 Missing ⚠️
src/gratingcalibration/beamstop_fitter.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #5       +/-   ##
===========================================
+ Coverage   34.09%   63.74%   +29.64%     
===========================================
  Files           8        8               
  Lines         569      673      +104     
===========================================
+ Hits          194      429      +235     
+ Misses        375      244      -131     

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