[feature request] Serialization & Weighted Values

[okayzed]

Two useful features for histograms are adding weighted values and the ability to serialize the sketch to and from bytes. Please consider adding these to DDSketch. I will attempt to add them in my own local fork of ddsketch as well.

The signature for adding weighted values is: addWeightedValue(val, count)

[jeerim]

Thanks for the suggestions. The sketches can now be serialized with protobuf, and weighted values can be added via AddWithCount(value, count).