example comparison to n-day historic values

Add an example to the function documentation that calculates the historical percentiles using a moving window. This will demonstrate the benefit of comparing the current year's daily streamflow to more than just that day's data.

Based on Scott's suggestion in !21 (merged):

What we can do though, to show the benefit of comparing the current year's daily streamflow to more than just that one day's data is to calculate the historical percentiles using a moving-window. That would be a great example to add too (but could do that in a separate PR)