Implement mean urine probe

[aegis301]

After discussion matters with some of our kidney experts there is a demand for a second method to calculate KDIGO urine output stages: Calculating the mean urine output over the given time window, dividing by the size of the window and then dividing by weight => calculating mean uo over time. This method might be less "strict" but also avoids under-classifying AKI. Users should have both options.