Metainfo versioning

As an requirement for effective #419 (closed) and for general future proofing, we should version the metainfo.

Ideally:

• different versions of the metainfo can be used within the same Python runtime
• NOMAD can use archives based on different metainfo versions
• the UI (metainfo and archive browser) are aware of different metainfo version

Problems to solve:

• metainfo packages have to state their version
• specific metainfo versions can be imported, e.g. through specific import mechanism
• metainfo packages of all available versions are packaged into nomad-lab Python package
• m_to_dict, m_from_dict and nomad.archive package write and read versions
• search and archive API respect versions

How can this ever work:

• Each Python module is its own namespace and imported names in different modules can come from different sources (e.g. versions).
• Python has a very flexible module and import system with lots of hooks
• Unfortunately, one can only hook into the first search for a python module/package name
• We internally add versions to the module name of metainfo packages
• Python modules can be "virtual", e.g. do not have to exist as files. We can implement finders/loaders/importers that resolve (versioned) metainfo package names