Add example of estimating variable streamflow percentiles

The current example of estimating a streamflow percentile for a streamflow observation makes use of the NWIS stats service. We should either replace this with using the hyswap functions or add an example for using hyswap.

Code in https://code.usgs.gov/water/computational-tools/surface-water-work/hyswap-example-notebooks/-/merge_requests/2 may be useful to pull from.