ENH: add 'dense' ranking method

[dsm054]

scipy.stats.rankdata has five ranking methods: average, min, and max, equivalent to the same options in Series.rank; ordinal, corresponding to our first; but it also has dense:

    'dense':
        Like 'min', but the rank of the next highest element is assigned
        the rank immediately after those assigned to the tied elements.

For example,

>>> scipy.stats.rankdata([3,1,2,1,3], "min")
array([ 4.,  1.,  3.,  1.,  4.])
>>> scipy.stats.rankdata([3,1,2,1,3], "dense")
array([ 3.,  1.,  2.,  1.,  3.])

I've wanted this in the past, and it just showed up on SO.

[jreback]

FYI, just merged this : #5978

I don't think it affects what you are doing..but be on the look out

[dan-blanchard]

👍

[jreback]

closed by #6514