Converts a point given in discrete coordinates withe respect to the
        grid in v to fractional coordinates.

def __d2f(disc_coords, v):
        """
        Converts a point given in discrete coordinates withe respect to the
        grid in v to fractional coordinates.
        """
        return np.array([disc_coords[0] / v.shape[0],
                         disc_coords[1] / v.shape[1],
                         disc_coords[2] / v.shape[2]])