Add functions for categorizing flow and improve percentile interpolation functions

This MR adds two utility functions that categorize streamflow based on defined schema. Default is set to mimic that of the National Water Dashboard, which is very similar to WaterWatch. This addresses #51 (closed)

MR also improves the usability of functions to estimate a percentile using existing streamflow percentile thresholds. Now options to calculating single and multiple percentiles for both fixed and variable percentile types. Addresses portions of streamflow percentile calculator #50 (closed)

Example added also closes issue #54 (closed)