Converts fractional coordinates to discrete coordinates with respect to
        the grid size of v

def __f2d(frac_coords, v):
        """
        Converts fractional coordinates to discrete coordinates with respect to
        the grid size of v
        """
        # frac_coords = frac_coords % 1
        return np.array([int(frac_coords[0] * v.shape[0]),
                         int(frac_coords[1] * v.shape[1]),
                         int(frac_coords[2] * v.shape[2])])