[WIP] Unify Random-Access API and Streaming API into Series::snapshots()

[franzpoeschel]

description soon

TODO:

• [x] Merge #1584 first
• [x] Better logic for READ_WRITE mode
• [ ] backwards compatibility: using definitions, empty headers
• [ ] documentation, change examples
• [ ] maybe snapshots(Snapshots::RandomAccess) vs snapshots(Snapshots::CollectiveAccess) or so
• [ ] Container API: How to deal with erased Iterations?
• [ ] snapshots().begin() -> reset to start, add sth like snapshots().current()
• [ ] Follow-up: Flush open iterations instead of dirty iterations.

Diff: https://github.com/franzpoeschel/openPMD-api/compare/shared-attributable-data...topic-iterator