detect externally-initiated slew when platform is tracking

[agalasso]

Slewing is not allowed when the platform is tracking. EQPAD will not allow slewing by clients, but it has no way to prevent an externally initiated slew, like from the hand controller. When platform motion stops we should validate that there has not been any slew--Alt and Az reported by the inner scope driver should have remained constant.
We could also periodically check the Slewing property of the inner driver to potentially detect the slew earlier (checking for change of alt/az when platform tracking stops would still be required)

[agalasso]

we should also validate that tracking of the alt-az mount had not been enabled externally while the platform is tracking