support different scale functions

[kat-grayson]

Hi, I am working on a project where we're looking for a fast implementation of the TDigest algorithm that will support streaming capabilities. Your project ticks both of those boxes! We are also looking for the ability to control the scale function used. I currently can't see which scale function has been used in your implementation. Firstly which scale function have you implemented and would you consider adding this an input option so the user would have control over the scale function?