detect and eliminate rain drop contaminated images

[RichardWagener]

rain drops on the lens cause the whole process to fail starting with cloud height determination, stitching, etc. If there is a way to detect and mask out raindrops there may be limited information in those images, but at a minimum we should be able to eliminate rain contaminated images altogether. Over our area not all cameras will be affected by rain at the same time.
Examples: 2018-09-11, 2018-08-01

[amcmahon01]

If this is predominantly an issue with overcast days, fixing the cloud fraction function to correctly handle overcast conditions may fix a majority of these cases.