Edit y-axis labels, correct cumulative hydrograph calculations

I changed the default y-axis labels from 'discharge, in cubic feet per second' to 'discharge, ft3/s' in the duration hydrograph plots. I converted cubic feet per second in the hyswap.cumulative.calculate_daily_cumulative_values function to acre-feet by multiplying by the number of seconds in a day and the conversion between ft3 and acre-feet. The examples generally match up with the WaterWatch image for the same gage (for WY 2020). The 25-75 percentile envelope is slightly different between the two plots, but I wonder if that is because the datasets used to create the percentiles are a little different (i.e. WW uses 2005 to 2023 and the example uses 2000 to 2022).

These edits should close issues #38 (closed) and #11 (closed)

Note that these changes also required some edits to the unit tests, where the tests expect a certain axis label and cumulative value.