Mcaliley review: frequency parameter documentation improvements

The docstrings for streamflow_to_runoff and convert_cfs_to_runoff say that the frequency parameter controls the frequency of the values to return. It controls the units of the runoff values returned, but it does not change the frequency in the sense of resampling the input time series, which would be one way of interpreting the docstrings. For example, if frequency='annual' and the input dataframe holds daily streamflow values, they get converted to runoff values with units of mm/yr but there are still values every day. I suggest revising the docstring to clarify that frequency controls the units of the return values. You are right, this is confusing. As a result, we have changed the input name to "time_unit", which is defined as the unit of time used in the returned runoff values following the format mm/<time unit>. Furthermore, "time_unit" now takes on the acceptable values of "year", "month", or "day", which matches the denominator of the unit and hopefully improves clarity.