add probability plots

[wholmgren]

Following the HRRRE methodology:

https://rapidrefresh.noaa.gov/hrrr/HRRRE/

“HRRRE probabilities are the fraction of members that exceed a given threshold, or predict a given precipitation type, at a point. The final probability field (100*(n/total)) is smoothed using a Gaussian filter of width 25 km.”

let's start with composite reflectivity > 50 dbz, then move on to hourly precip, then 24 hour precip. Might want to do precip in a separate pull request due to the time complication.

@lboeman I will sketch out the algorithm for this and then leave it to you to move to production.

[lboeman]

@wholmgren Does this still need to be done?

[wholmgren]

yes, but it is low priority right now