Event List Context Data Model

[mservillat]

https://github.com/ivoa/HighEnergyDataNote/blob/main/figures/EventListContext.png

General comments to update this diagram :

• Observation->EventList multiplicities should be 1->*
• StableTimeInterval : change to TimeInterval and add type=Good/Stable
• TimeInterval->IRF multiplicities should be 1->* (e.g. GTIs can share the same IRFs, but STI have dedicated IRFs)