add monitor to check for the addition of new channels within frames

There are certain processes that may benefit from knowledge of whether or not new channels appear in the frames. This could be done with a simple script running as a Nagios plugin that does something like

FrChannels $(gw_data_find -o L --type L1_R -u file --latest) | sort | uniq > new.out
if [ "$(ls old.out)" -n ] ### check to see if we have an existing list of channels
then
  if [-n "$(diff new.out old.out)" ]
  then
    returncode=1 ### report something to Nagios to say there was a change in the channel list
  else
    returncode=0
  fi
fi
cp new.out old.out

As long as it's run repeatedly in the same directory, this or something similar should work. We should run it for each IFO separately, and possibly for multiple frame types, but the script should be recyclable for those purposes.