add Shading, Soiling, and Reflection Losses

[dacoex]

These seem to miss on the environmental input side:
https://pvpmc.sandia.gov/modeling-steps/1-weather-design-inputs/shading-soiling-and-reflection-losses/

Maybe this meta issue can track this as mentioned in
#2 (comment)

[wholmgren]

PRs for shading and soiling models are welcome. To my knowledge, this is not something that's currently in the Matlab project.

There are already two reflection loss functions: pvsystem.physicaliam and pvsystem.ashraeiam. Did you want something different?

[cwhanse]

I think this can be closed as a general issue, with #860 #669 #1017 and all of pvlib.iam.