Writing and Reading Non-openPMD Infos

It would be necessary to allow storing non-openPMD information as well.

In the simple case, this means adding additional, user-defined attributes to records, record-components and paths.

In the more complex case, application specific states such as RNG states, unqiue-number-generator states, etc. need to be stored as records outside of the openPMD-interpreted paths.