fix mlm:artifact_type check missing in assets and other disallowed MLM fields

[fmigneault]

Description

• Improve the README regarding allowed/disallowed fields in the Assets vs Item vs both.
• Align the allowed/disallowed fields behavior with the JSON schema
• Ensure that, for the Asset that contains the mlm:model role, the mlm:artifact_type is also provided
• Add multiple tests validating above behaviors

Related Issue

• fixes Fix missing mlm:artifact_type in JSON schema #42
• closes fix mlm:artifact_type check missing #52
• closes fix mlm:artifact_type check missing + update corresponding tests/examples #58

Type of Change

• 📚 Examples, docs, tutorials or dependencies update;
• 🔧 Bug fix (non-breaking change which fixes an issue);
• 🥂 Improvement (non-breaking change which improves an existing feature);
• 🚀 New feature (non-breaking change which adds functionality);
• 💥 Breaking change (fix or feature that would cause existing functionality to change);
• 🔐 Security fix.

Checklist

• I've read the CONTRIBUTING.md guide;
• I've updated the code style using make check;
• I've written tests for all new methods and classes that I created;
• I've written the docstring in Google format for all the methods and classes that I used.

[rbavery]

Look great overall, one question on if we should mark a couple of other fields as only Item properties.