Write a user defined function FindFrequency which inputs are a time and a wave elevation vector and output the approximated wave frequency (fappr=FindFrequency(t,eta)). The function should find the different intervals [tn tn+1] where the free surface changes sign. Then, it should calculate the coordinates of the points (((tn + Tn+1)/2),0), plot the points and return the average of the approximated frequency.