Wppf weights

[saransh13]

Overview

This branch adds statistically informed weighting for WPPF refinements of azimuthally averaged polar data.

• Adds N_valid(pv) to count valid pixels contributing to each polar lineout point.
• Adds optional N_sampling=None arguments to LeBail and Rietveld.
• When supplied, scales WPPF weights using the number of contributing samples.
• Supports masked one-dimensional N_sampling arrays across disconnected valid regions.
• Separates reduced χ² from goodness of fit:
  • chi2 = weighted_sum_squares / (N - P)
  • gofF = sqrt(chi2)
• Updates WPPF output to report S (goodness-of-fit) instead of χ².
• Changes calc_rwp() to return both chi2 and gofF.

Affected Workflows

LeBail and Rietveld refinements can now account for the number of detector pixels averaged into each lineout point. Existing callers remain compatible because N_sampling defaults to None, preserving the previous weighting behavior.

Callers unpacking calc_rwp() directly must account for its additional return value.

Documentation Changes

The LeBail and Rietveld parameter documentation now includes N_sampling.

Testing

No tests are added on this branch. Recommended coverage includes:

• Valid-pixel counting for masked and NaN-containing polar images.
• LeBail and Rietveld behavior with and without N_sampling.
• Masked N_sampling region handling.
• Independent χ² and goodness-of-fit calculations.

[codecov]

Codecov Report

❌ Patch coverage is 57.62712% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.38%. Comparing base (073fd14) to head (30529c6).

Files with missing lines	Patch %	Lines
hexrd/powder/wppf/WPPF.py	57.50%	17 Missing ⚠️
hexrd/core/projections/polar.py	27.27%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #986      +/-   ##
==========================================
- Coverage   75.45%   75.38%   -0.07%     
==========================================
  Files         149      149              
  Lines       22961    23000      +39     
==========================================
+ Hits        17325    17339      +14     
- Misses       5636     5661      +25     

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