Roadmap for further NIX/odML development

NIX should be the umbrella joining odml with data.

Aiming to solve some issues in odml 1, move it closer to rdf compatibility and allow for seamless switch/integration of existing to nix.

1. Make the python lib python 3 compatible.
2. Separate the editor from the core lib.
3. Add UUIDs to entities.
4. Add (optional) field for predicate defining the link between subsections.
5. Do not allow multiple values.
6. Remove unused Value fields: checksum, filename, encoder, reference.
7. Reference field in sections does define something that is related to the section.
8. Remove the mapping stuff.

This will lead to version 1.5(?!) of odml.

Same changes will be done to the NIX implementation of the model to make them 100% compatible.

Extract the odml part of the nix library -> odml_core lib with multiple backend optionality. Extend the core library to enable odml files to act as a hub to refer/link to data files. E.g. linking into nix files.

Problemsets:
- Harmonization of the odml implementations.
- Writing between implementations → read odml from one implementation and integrate into the other.
- Reading nix/odml in a joint dom tree.
- Editing nix/odml originating metadata in a joint dom.