Add running monthly CMD sums through entire normal periods

[whmacken]

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.

[kdaust]

BG, SBSmc2, ICHmw2, CWHvm1, GBD_xh_NV 61 - 90 period