EventLoaderCLICpix2: fix calculation to set timestamp half way between the shutter open and close

Calculation of the default pixel timestamp in EventLoaderCLICpix2 was incorrect. It was:

double timestamp = (shutterStartTime - shutterStopTime) / 2;

and now it is in the middle of the frame as wanted:

double timestamp = shutterStartTime + (shutterStopTime - shutterStartTime) / 2;