Add preferred-orientation (March–Dollase) correction for textured powder samples

[AndrewSazonov]

Powder samples whose crystallites pack with a preferred orientation — platy or needle-like grains — no longer have to be modelled as ideal random powders. You can now add a March–Dollase preferred-orientation correction to each phase of a constant-wavelength Bragg neutron experiment: set the March coefficient and a crystallographic direction, optionally a random-fraction term, then refine them alongside the scale to improve the fit. The setting is saved to and restored from your project CIF and appears in the IUCr-aligned report. Corrections run on the CrysPy engine, and a new cross-engine verification example built on the LBCO dataset demonstrates that the refined result reproduces the equivalent FullProf March–Dollase correction.

[codecov]

Codecov Report

❌ Patch coverage is 93.07692% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.26%. Comparing base (beba720) to head (680fdea).

Files with missing lines	Patch %	Lines
src/easydiffraction/analysis/calculators/cryspy.py	73.52%	4 Missing and 5 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #200      +/-   ##
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+ Coverage    90.22%   90.26%   +0.03%     
===========================================
  Files          271      274       +3     
  Lines        23273    23403     +130     
  Branches      2718     2729      +11     
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+ Hits         20999    21124     +125     
+ Misses        1707     1705       -2     
- Partials       567      574       +7     

Flag	Coverage Δ
unittests	90.26% <93.07%> (+0.03%)	⬆️

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Files with missing lines	Coverage Δ
...ocks/experiment/categories/pref_orient/__init__.py	100.00% <100.00%> (ø)
...locks/experiment/categories/pref_orient/default.py	100.00% <100.00%> (ø)
...locks/experiment/categories/pref_orient/factory.py	100.00% <100.00%> (ø)
...easydiffraction/datablocks/experiment/item/base.py	95.83% <ø> (ø)
...diffraction/datablocks/experiment/item/bragg_pd.py	95.23% <100.00%> (+0.30%)	⬆️
src/easydiffraction/io/cif/iucr_writer.py	96.51% <100.00%> (+0.06%)	⬆️
src/easydiffraction/analysis/calculators/cryspy.py	45.28% <73.52%> (+2.47%)	⬆️

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