The _percentile function that iris.analysis.PERCENTILE is based on allows the keyword fast_percentile_method so that np.percentile may be used in place of scipy.mstats.mquantiles. However, this option does not appear in the documentation for iris.analysis.PERCENTILE. [Addresssed in #3901]

Also, is there any reason why the _percentile function and PERCENTILE aggregator shouldn't allow passing of **kwargs to np.percentile if this option is chosen? If implemented, this would allow users to specify an interpolation method in np.percentile. [Addressed in #4791]