Add formula to mean_pressure_weighted function

As noted in this [Stack Overflow question](https://stackoverflow.com/questions/69045234/calculation-of-the-mean-pressure-weighted-function/69123801?noredirect=1#comment122204804_69123801) it would be good to be clear what’s going on in this function, including the equation.