Define format to address the various physics cases

[ramirezdiego]

See #41, where @HenningSE propagated an epix tag to filter only certain events (clean-ish NR energy deposits). We might want to give this a proper structure for other cases (multi-site calibrations, er_only, etc.).

[HenningSE]

Based on some discussions on slack, one use case could be an energy cut. We could select only events with a sum energy in some given energy range.