Add lambertw_pvlib

[cwhanse]

• Closes #xxxx
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• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
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Add a real-valued-only Lamberts W function. As tested, function is as precise as scipy's lambertw, and 4x faster due to not doing all arithmetic with complex values.

[cwhanse]

Once this is accepted, next PR will replace calls to scipy's lambertw in ivtools. I'm splitting this in two parts, since it's not a matter of replacing a function name. The private function _log_lambertw handles the overflow cases that are computed using a Newton's technique in the ivtools functions.

[cwhanse]

Put this in pvlib.tools or in pvlib.ivtools.utils? Make lambertw_pvlib private?

pvlib docs don't provide links to public function in either of these folders so there's no API change.