Feature Request: soiling model

[DanRiley]

A soiling model was presented by Merissa Coello and Liza Boyle from Humboldt University at PVSC/WCPEC 2018. Requires time-series data of particulate matter (PM) concentrations for 10 micron and 2.5 micron particle diameters, and rainfall. Other parameters needed for variable deposition models. I have a copy of the model paper from Merissa and Liza, but can't find a reference to the proceedings. I have a model function developed and am putting final touches on the model. I'll PR when complete.