Update to reflect new changes to the t-digest algorithm

Lately there have been many changes to the t-digest that improve both accuracy and speed of the algorithm. Having the changes incorporated into this module will be very useful when this implementation of t-digest is used to implement the user facing `quantile` function in Dask.

There are couple of questions open still before the work can begin:
1. Is the algorithm described in [this](https://arxiv.org/abs/1902.04023) paper the final one?
2. Does the Java implementation in the [t-digest repository](https://github.com/tdunning/t-digest) contain the new changes? Can it be used as a reference?