Bifacial option in Mermoud Lejeune module model

[allenlawrence94]

Issue: #241

I cherry-picked the commit that exposes a bifacial option from @caseyzak24's PR #314, added a test, and fixed a couple bugs.

More detail on the bugs:

• There was no calculation of nominal module efficiency in the Mermoud case, which created nonsense results for nominal dc energy and performance ratio when using the Mermoud model.
• IAM would evaluate to nan when front-side irradiance was 0. It seems sunup should prevent the pv model from running when irradiance is 0, but it still happens occasionally (in the new test case it occurs in 112 shoulder hours in the year). Is this okay?

[janinefreeman]

@allenlawrence94 Thank you so much, this is great! I will be merging this in for our upcoming release! We super appreciate the contribution.

[allenlawrence94]

Awesome! Thanks janine