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

Morag Jean Williams requested to merge williamm/corryvreckan:master into master

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;

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