Add signal to Gen2 indicating the completion of an exposure

We have added a signal passing the SPS shutter state to Gen2, and understand how to use that in the GETOBJECT skel file: when that goes CLOSED the telescope can be slewed, etc. This is all done while the exposure command is still running.

But we also need a signal for when the exposure command finally completes. In practice this may not be absolutely necessary: any real slew+convergence will take longer than any readout. It is certainly not necessary to test/implement slewing/converging-while-reading-out.

But there may well be cases where it does matter (oh, 4-channel H4 readouts was a recent possibility.) So add one.