Aggregate multiple input data sets

For the image binner `images` (and also for the putative new time histogram tool, see #21), allow the user to pass multiple input events data sets and aggregate them, to increase the signal in the output data. This will require handling the cues and events separately for `--align-trigger`, `--trigger-type`, etc., as well as checks for compatible `--image-size`, etc., and then aggregating the events in the `bincount` step.