Support events and periods

[nichtich]

A concept can have events and periods such as death, profession, school...

• type (TODO: which URI as mandatory?)
• name/label
• date (startDate/endDate or relatedDates?)
• textual description
• place
• related entity

Question: add a new field such as "events" and/or require a type such as E5_Event.

[nichtich]

DANTE also has some independent chronologies e.g https://uri.gbv.de/terminology/nld_chronology/ but this issue is about event records as part of other records withut assigning the events to a vocabulary of its own.