Add option to consider event independence based on X-day gap

[jds485]

Several studies employ a check that unique events are separated by more than X days, where X can be user defined. EflowStats does not currently have this option. We can work with EflowStats to add this option, or add it ourselves.

If the gap is shorter than X, we could merge events into 1 event (and fill in the X day gap as part of the event).

For the paper, we would use this to check if defining events with vs. without an X day gap makes a difference for cluster regions.