Implement mappings to other metadata standards

[Ludee]

Description of the issue

Add mappings and documentation.

Ideas of solution

• DCAT-AP
• INSPIRE
• DataCite
• RO CRATE
• Zenodo
• SKOS

Workflow checklist

• I am aware of the workflow in CONTRIBUTING.md

[jh-RLI]

We will add a new module called src/omi/crosswalk.py and @vismayajochem will start implementing the code structure.

We should also figure out how to parse and navigate the different file formats in which metadata is available