Sun elevation filter

[williamhobbs]

In other performance models using measured data (not rdtools), I have found that that filtering out periods where sun elevation is less than 10 to 20 degrees above the horizon can be helpful, as it reduces periods with row-to-row shading and near object/horizon shading of modules or pyranometers. This is in addition to a POA filter.

Based on a few very quick tests with rdtools, I see a slight reduction in uncertainty when adding this as a filter.

Would adding this as a default or option be worthwhile?