Kill everything called by the script when pressing the "stop" button

[rdrighetto]

Whenever the user interrupts a script by clicking the "Stop" button, it should automatically kill all programs called by that script (e.g. Python scripts, FREALIGN instances). I know this can be very tricky and highly OS-dependent, but it would be really useful. For example, at the moment, the particle picking Python script can take a long time to run, and if the user stops the CSH script from the 2dx GUI, it keeps running the .py script in the background. This is very annoying and resource-consuming.