[GA-1599] additional humid heat metrics

[kafitzgerald]

From our suggestion box:

...other humid heat metrics could be helpful as well (sWBGT, WBGT, etc. as these are standard for ILO, other large reports on humid heat impacts on global labor). Buzan et al., 2015 came up with a coding package to do this for CLM I think. More recent work (e.g., Li et al., 2020, Env. Res. Lett.) calculated sWBGT from CESM Large Ensemble data using Python, so the coding framework is out there (Li et al shared a link to their Python code here: https://github.com/dw-li/WBGT).