Add Wavelet Variability Model (WVM) for calculating spatial smoothing of irradiance

[jranalli]

Should I spin this off to a separate issue, since it might be different (and more compartmented) than the broader downscaling discussion?

Yes. Let's start a new module with this submission, scaling.py comes to mind, but I'm not enamored of it. Scope will be functions that operate on irradiance, perhaps other variables, to transform temporal or spatial characteristics.

Spinoff from issue #788 . Implementation is a python port of WVM, released as an auxiliary to the Matlab pvlib here. My implementation ports the original model logic, but deviates from the overall package, in that I begin at the point where the user already has a clear sky index to operate on (original starts from GHI and calculates POA clear sky index). I thought this would allow for more flexibility in choice of transposition model, etc, but it does ask a bit more work up front for a user to run the WVM.

I am close to completion of a draft and will create a pull request when ready. This is my first contribution to the project (or any open source project really), so please accept my apologies in advance if it takes some guidance.

[cwhanse]

This is my first contribution to the project (or any open source project really), so please accept my apologies in advance if it takes some guidance.

Welcome! Asking for a clear-sky index as input seems appropriate; there's no need to rigidly follow the MATLAB implementation. I'll ask for your patience with the review process, which can involve multiple iterations and reviewers.