Linear shade loss model - continuation of pr 1725

[echedey-ls]

• Closes Add linear shade loss model for thin film #1690
• Substitutes Linear shade gh1690 #1725 (originally from @mikofski )
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• Updates entries in docs/sphinx/source/reference for API changes.
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Addresses the linear thin film module losses.

Comment from @kandersolar at #1962:

you are saying that we need a substantive reference that says linear power loss models are valid for thin film modules. Is that right?

What I'd really like to reference is a paper that deals with specifically the equation implemented in this PR: power_loss_fraction = shaded_fraction1d * (1 - diffuse_fraction).

@mikofski do you have a reference for that or should I change to irradiance losses and a SAM reference?

Wait for #1962 to be merged before this one so the docstring makes sense.