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

@classmethod
def from_cartesian(cls, cart):

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