Dry spells calculation for CSM

[drewmresnick]

          For dry spells defined as Tufa did, there is longest_run_length that could be a source of inspiration. Instead of taking the max at the end, could divide by the length that defines a dry spell, that would give a count of spells within a series of continuous spells, and then sum that to count all the spells in the season. I think... need to check what goes into the max is what I claim here, it's been a while...

Dividing (or using modulo) should be done on integers so that, if the threshold for spells is 5 days, 6 will return 1, 14 will return 2, etc.. which sounds appropriate.
Now there could be an approach outside longest_run_length that is better but I'd have to think more about it.

Originally posted by @remicousin in #280 (comment)