Filter dNBR

Probably in the severity module.

1. Optionally apply 5x5 median focal stat
2. Set anything <= -2000 to NoData
3. Apply a 20x20 maximum focal stat - save this
4. Set NoData values to the value from the 20x20 focal stat (data values remain as they are)
5. Set values <=-1000 to -1000
6. Set values >=1000 tp 1000