I updated the IMC_LOCK guardian to make the ISS_ON state more reliably accessible when the IFO is locked.  Much of this was just moving some code from CLOSE_ISS.main to CLOSE_ISS.run, and using cdsutils.avg to acquire data.  I also created several helper functions to abstract some of the ISS switching.

The current guardian ISS enabling code has been tested a few times at high power, and it has worked without trouble.  One new feature is that the IMC guardian keeps the second loop output value small, averaged over long times.  This is a sort of "digital AC coupling" which may prevent problems with ISS actuator saturation over long locks.

With the ISS on, the DARM noise is significantly improved between 100 and 400Hz.  It doesn't seem that more gain is necessary at this point.