Add openPMD version 2.0 to supported API versions

[franzpoeschel]

This does not yet add any openPMD 2.0 functionality, but serves as a basis for PRs that add new functionality.

This does not yet set 2.0 as the default written version, hence getStandard() needs to be split into getStandard() and getStandardDefault().

• [x] Maybe add getStandardMax() additionally and deprecate getStandard()?
• [x] issue warning that 2.0 is in development

[ax3l]

Sounds good :+1:

We could also separate into default/min/max standard for read and write.

[franzpoeschel]

Marking this as WIP since I'm using this PR as a common ground for new openPMD 2.0 features and might want to extract more features into it in future.