Fix and Extend set_HAP_refinements() for Size and Mustrain Parameters

[AdamCorrao]

Summary

This PR enhances the G2Phase.set_HAP_refinements() method in GSASIIscriptable to properly handle Lorentzian-Gaussian mixing coefficients (LG_mix) for Size and Mustrain parameters, and fixes a bug when setting uniaxial Size and Mustrain values.

Changes to GSASIIscriptable.py

Bug fix:

• Value setting for uniaxial 'Size' and 'Mustrain'

New functionality:

• Added LG_mix parameter handling for both Size and Mustrain HAP parameters
• Users can now set both the value and refinement flag for LG_mix coefficients
• Included validation to ensure LG_mix values are between 0.0 and 1.0

Example usage:

phase.set_HAP_refinements({
    'Size': {'type': 'isotropic', 'value': 0.1, 'refine': True,
             'LGmix': {'value': 0.5, 'refine': True}},
    'Mustrain': {'type': 'isotropic', 'refine': True,
                 'LGmix': {'value': 0.8, 'refine': True}}
})

Testing Materials

Added testing materials in test-G2sc-size-strain/ directory:

• Notebook: G2sc_size_strain_test.ipynb - demonstrates the new functionality with a complete workflow
• Test data: LaB6 standard reference material (SRM 660c) diffraction pattern
• Supporting files: CIF file and instrument parameter file for reproducible testing
• Workflow: Shows project creation, histogram and phase addition, Size/Mustrain parameter setting, and refinement execution

The test notebook validates that the enhanced set_HAP_refinements() method correctly:

• Configures refinement flags for Size and Mustrain parameters
• Sets and refines LG_mix coefficients for peak shape modeling
• Integrates with the full GSAS-II refinement workflow