add NASA-NEX modification to bias correction options 

this method is a slight modification to the classic bias correction method in that it fits quantile maps by day of year using day groups composed of the day of year +/- 15 days, see: https://www.hydrol-earth-syst-sci.net/16/3309/2012/ and https://www.nccs.nasa.gov/services/data-collections/land-based-products/nex-gddp