AVLTreeDigest can have centroids out of order

[tdunning]

The test com.tdunning.math.stats.TDigestTest#testSorted can fail with out of order centroids:

previous = {Centroid@1053} "Centroid{centroid=0.9999500019381296, count=5}"
 centroid = 0.9999500019381296
 count = 5
 id = 370
 actualData = null
centroid = {Centroid@1054} "Centroid{centroid=0.9999500019381294, count=3}"

Replicate (sometimes) with

mvn test -Dtests.seed=B382E9EF2739FED -DrunSlowTests=false -Dtests.class=com.tdunning.math.stats.AVLTreeDigestTest