Townsend snow (building on #1251)

[reepoi]

This builds off of pull request #1251 to add the changes suggested in the conversation there and wrap it up.

• [x] Closes #1246
• [x] I am familiar with the contributing guidelines
• [x] Tests added
• [x] Updates entries in docs/sphinx/source/reference for API changes.
• [x] 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:`user`).
• [x] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• [x] Pull request is nearly complete and ready for detailed review.
• [x] Maintainer: Appropriate GitHub Labels and Milestone are assigned to the Pull Request and linked Issue.

[cwhanse]

A communication with the model author T. Townsend: "the pile of snow was modeled as a triangle atop a rectangle, with yes, a vertical wall on the "north" side of the pile. This assumption, while an engineering approximation, is based on observing the shape of shedded snow piles, piles that are shaped by the slight forward (i.e., southward) velocity of the sliding/falling snow and ice."