Updates to munge_nwis_stats utility function

This MR revises the munge_nwis_stats utility function to by default retrieve all percentile threshold levels and to index only by month-day. Previously, a multi-index of month-day and day-of-year was used which results in issues with doy and month-day not being correctly aligned due to leap days. Now no day-of-year column is calculated or returned in the dataframe. Relates to #61 (closed) and !56 (closed)

Revisions here should assist with validation testing and comparison of hyswap calculations to the current nwis stats service (which is based on month-day).

Closes issue #64 (closed)