Calculates mean intensities of pixels in superpixels
    inputs:
        im ... grayscale image, ndarray [MxN]
        suppxls ... image with suppxls labels, ndarray -same shape as im
    outputs:
        suppxl_intens ... vector with suppxls mean intensities

def get_superpxl_intensities(im, suppxls):
    """Calculates mean intensities of pixels in superpixels
    inputs:
        im ... grayscale image, ndarray [MxN]
        suppxls ... image with suppxls labels, ndarray -same shape as im
    outputs:
        suppxl_intens ... vector with suppxls mean intensities
    """
    n_suppxl = np.int(suppxls.max() + 1)
    suppxl_intens = np.zeros(n_suppxl)

    for i in range(n_suppxl):
        sup = suppxls == i
        vals = im[np.nonzero(sup)]
        try:
            suppxl_intens[i] = np.mean(vals)
        except:
            suppxl_intens[i] = -1

    return suppxl_intens