Return period method

[MAfarrag]

Is your feature request related to a problem? Please describe.
create a return period method for each distribution using the sf method in the scipy distributions, rather than the method i implemented with my own calculations.

return_period = 1 / genextreme.sf(700, shape, loc=loc, scale=scale)
print(f"Return Period for flow of 700 units: {return_period} years")