Add guidance or modified Townsend model for snow loss with bifacial arrays

[cwhanse]

T. Townsend recently published guidance for applying the Townsend snow loss model to bifacial arrays, in the technical document linked here.

1. does this guidance describe a modification in the Townsend monthly loss model at pvlib.snow.loss_townsend, or
2. does the guidance describe how to apply the existing Townsend model to bifacial systems?
3. do we consider the technical note citable?
4. if the note is citable then create a follow-on issue to modify the model, documentation or add an example, as determined by the answers to 1 and 2 above.