Add running monthly CMD sums through entire normal periods

A couple of more complicated variables for CDM and heat drought for climr.

- [x] Calculate a running monthly CMD sum. e.g. CMD.sum_03 = CMD01+CMD02 +CMD03 through the entire normal period. At CMD 10 reset any value greater than 500 to 500.
- [x] Reduce end of season CMD by accumulated non-growing season precipitation. month 10-04. (might modify this) based on DD5 sum
- [x] Annual CMD max
- [x] Annual variable months in drought (CMD_max > 600). Normal period statistics
- [ ] Cutoffs: 600, 800, 1000, 1200
- [x] Running 3 year sum of max. Sum CMD.max of year-2 +year-1+year0
- [x] Flag months with potential Heat-Drought where month CMD.sum >600 and Tmax >30.
- [ ] Heat drought severity rating matrix to attribute month
- [x] Annual variable: months with heat drought. Normal period statistics.

Start by locating the package code that calculates CMD sums and heat-drought variables, then compare the checked and unchecked items in this issue with the current implementation. Clarify whether the remaining work is the cutoff variables, the heat-drought severity matrix, or both, and confirm the expected normal-period statistics before changing anything.

Written by the indexing model from the issue text.