Resolve "Split `fgen.models`"

Description

• split out fgen.models into smaller files so it's easier to navigate
• split out fgen.models.serialisation so the data models code is separate from the serialisation code

Doing this before #53 (closed) because order actually doesn't matter I realised

Checklist

Please confirm that this pull request has done the following:

- [ ] Tests added

• Documentation added (where applicable)
• Changelog item added to changelog/

Closes #54 (closed)