Bug#784947: libt-digest-java: Version 3.1 is severely incompatible with version 3.0

[Emmanuel Bourg]

I got a look at Elasticsearch, and this is just a one line change:
state.centroidCount() becomes state.centroids().size() in
TDigestState.java. I wouldn't call that a severe incompatibility.

We can also reintroduce the missing centroidCount() method in t-digest,
such that our package supports rdeps expecting the version 3.0 or the
version 3.1.

Emmanuel Bourg