Add bifacial scaling to Townsend snow loss model

[shethkajal7]

Add optional front_side_fraction argument to loss_townsend for bifacial snow loss adjustment. Includes test coverage for the scaling behavior.

Description

This PR adds an optional front_side_fraction argument to pvlib.snow.loss_townsend.

The new argument allows users to scale the calculated Townsend snow loss by the front-side energy fraction for bifacial systems. This follows the bifacial guidance in Townsend and Previtali (2023), where bifacial snow modeling uses total front + rear POA in the Townsend calculation and then scales the resulting loss by the front-side energy contribution.

The default value is None, so existing behavior is unchanged.

Changes

• Add optional front_side_fraction keyword argument to loss_townsend
• Document bifacial usage in the function docstring
• Add test coverage confirming that front_side_fraction scales the existing Townsend result correctly

Backward compatibility

This change is backward compatible. When front_side_fraction=None, the function follows the existing behavior.

Tests

Ran locally:

python -m pytest tests/test_snow.py -k townsend
python -m ruff check pvlib/snow.py tests/test_snow.py

Both passed.

Checklist

• Closes Enhancement to Townsend monthly snow loss model for engineering workflows #2755
• I am familiar with the contributing guidelines
• I attest that all AI-generated material has been vetted for accuracy and is in compliance with the pvlib license
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`shethkajal7`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.