Requirements for Extensions

[ax3l]

Coupled of from the discussion with @C0nsultant in openPMD/openPMD-api#283

We should state in the base standard, where we introduce extensions that extensions must be compatible with the base standard in the following way:

• redefining meaning is fine (e.g. positions), but keep logic (e.g. summation rule in positions Rename position and positionOffset #182)
• removing (required) attributes or (required) base structure is not allowed
• adding things is fine (attributes, extra structure that does not break base parsing, etc.)
• ...

Otherwise a base standard makes no sense, because the idea is that general tools/readers/writers can be designed that can read all openPMD data (maybe just having less context if the extension is unknown).

We do this already for our official extensions because it is the logical way to do things, but we don't mention (enforce) it yet in the base standard.