Observing system

A known problem with NOTCam is that sending an abort signal to the detector controller during an on-going exposure will cause the system to hang, requiring a restart of the system. It was noted that the `abort' command has been disabled, but that doing a Ctrl-C when running a script can cause a problem when given at the wrong time. To avoid this, basically all commands that perform an exposure (exp/dark/frame/dframe) were modified such that it will ignore any Ctrl-C command (with an appropriate warning message in the Talker) when given during the execution of an exposure.