Handle Ctrl+C signal

[maxbachmann]

Since some of the processor functions can run for a long time. However it is currently not possible to quit by pressing Ctrl+C. Instead it is required to manually kill the process. The implementation should periodically acquire the GIL and check _PyOS_InterruptOccurred, to see whether a KeyboardError occured.

It would be fine to check this e.g. only once per second to keep the performance impact low, but the application should eventually stop after Ctrl+C is pressed.