Add ability to save diagnostics at particular percentiles/quantiles

[ph-kev]

Calibrating climate extremes would involves statistics about the percentiles or quantiles of variable of interest. For example, you might want to calibrate on the 90th and 10th percentile/quantile of a variable.

This would involves implementing a streaming algorithm that can compute percentile / quantile as it is not feasible to store every single sample to compute the statistics at the end of the reduction period.