Add engerer2 decomposition model

[AdamRJensen]

Is your feature request related to a problem? Please describe.
The engerer2 separation/decomposition model generally outperforms all of the decomposition models available in pvlib according to https://doi.org/10.1016/j.solener.2015.10.010.

Describe the solution you'd like
Add a Python implementation of the engerer2 model to pvlib.

Additional context
A Python implementation is available from the author, though there is no license information given. @JamieMBright?

This could potentially be labeled a good first issue.

[kandersolar]

Closing, see comments in #1366