Have TDigest implement Consumer

[alexec]

I'm finding myself wanting to use TDigest with Java streams. I'd like to have a Collector for Digests. This would make my code simpler and more eligant.

[alexec]

    public static final Collector<Double, TDigest, TDigest> TDIGEST_COLLECTOR = Collector.of(
            () -> TDigest.createDigest(100),
            TDigest::add,
            ( digest1,  digest2) -> {
                digest1.add(digest2);
                return digest1;
            }
    );