More statistics from tdigest

[jcrist]

There are a few more statistics we could be getting out of a tdigest:

• [ ] sum (exact. Defined as sum of the values)
• [ ] count (exact. Available currently as size, defined as sum of the weights)
• [ ] mean (exact. Defined as sum/count, equal to mean if of values if all weights are 1)
• [ ] trimmed_mean (approximate. See paper)