Add factor to scale the length of the arrows in vector field

[superstar54]

phonon_setting = {"eigenvectors": np.array([[[0, 0], [0, 0],[0.5, 0]],
                                        [[0, 0], [0, 0], [-0.5, 0]]]
                                       ),
            "kpoint": [0, 0, 0], # optional
            "amplitude": 5, # scale the motion of the atoms
            "factor": 1.5, # scale the length of the arrows
            "nframes": 20,
            "repeat": [4, 4, 1],
            "color": "blue",
            "radius": 0.1,
            }