'Converts 3D rectangular cartesian coordinates to cylindrical polar
coordinates.'

@classmethod
def from_cartesian(cls, cart):

    rho = np.hypot(cart.x, cart.y)
    phi = np.arctan2(cart.y, cart.x)
    z = cart.z
    return cls(rho=rho, phi=phi, z=z, copy=False)
