could not reference motor.user_readback in swait calc

[prjemian]

Using this code to set up a swait record as a noisy detector:

APS_devices.setup_lorentzian_swait(
    calcs.calc1,
    m1.user_readback,
    center = 2*np.random.random() - 2,
    width = 0.015 * np.random.random(),
    scale = 10000 * (9 + np.random.random()),
    noise=0.05,
)

This failed when the EpicsSignalRO failed the test of isinstance(m1.user_readback, EpicsSignal)