Tdigest

- Fixes pickling error when using PySpark.

- added new methods onto `TDigest` object:
- `to_dict()`: return a Python dictionary of the TDigest and internal Centroid values.
- `update_from_dict(dict_values)`: update from serialized dictionary values into the TDigest object.
- `centroids_to_list()`: return a Python list of the TDigest object's internal Centroid values.
- `update_centroids_from_list(list_values)`: update Centroids from a python list.

- updated to use new accumulation_tree library. This replacement means much better performance in updating the TDigest.
- TDigest is Python3 only going forward.
- Fixed issues with `percentile` function
- `quantile` has be renamed to the more correct name `cdf`

- fixed a bug in `trimmed_mean` that was giving incorrect results. Proper interpolation and tests correct this.

- new dependency updates.
- fix quantile with only one centroid.

- The definition of percentile was used incorrectly in tdigest. Previous to 0.3.0,
a percentile was defined between 0 and 1. In fact, a percentile is defined between 0 and 100 (hence the 'percent' in percentile). This follows other conventions, like in Numpy and Scipy. This affects the `TDigest.percentile` function.