def deriv(self, m):
        return Utils.sdiag(np.exp(Utils.mkvc(m)))
histo (1-d array, giving the number of pixels in each quantized
dataset/ETHPy150Open simpeg/simpeg/SimPEG/Maps.py/ExpMap.deriv--crossover with--dataset/ETHPy150Open fengsp/color-thief-py/colorthief.py/MMCQ.get_histo