Where to put livetime info in IACT DL3?

[cdeil]

@contrera @jknodlseder @cboisson @kosack, all - Where should we put livetime info in the IACT DL3 model / formats?

At the moment LIVETIME (and ONTIME and DEADC) are required keywords in the EVENTS header (see here).

Is that a good idea?

Other options might be:

• As a column in the GTI table, i.e. have a livetime per GTI interval
• As a column in the POINTING table, i.e. introduce START, STOP intervals there have have LIVETIME be associated with those intervals. This is what Fermi-LAT does in their SPACECRAFT tables, see here and discussion here (where @jknodlseder and @kosack said they don't like this option).
• As a column in a to-be (not exising yet) "IRF validity time interval"

I think having it as an EVENTS header keyword is maybe sub-optimal, since (I think) it prevents merging of event lists and also it might not be the natural place if time information (like GTI or "response time interval") is elsewhere.

Livetime info is central to any science tools analysis, this is something we should do well in the DL3 data model.

Let's use this issue to gather some ideas and then discuss it tomorros in the telcon.