Extrema

my_extrema finds the positions of the extrema in each matrix column.

MY_extrema finds positions of extrema in each column of matrix "mat" and returns "pos_extr" matrix of the same size as "mat" with -1 at the minima's position, 1 at the maxima's position, 0 lsewhere. The search takes results from the test of the sign hange in the corresponding derivation of the "mat". The search technique is adapted for fft specta as the function argument.The pectrum repetition of "mat" is taken into account y extrema test on the edges. The script does not deal with NaN.
In case of flat extrema, the last position on the flat is treated.