Convert from biolink-model-pydantic to linkml pydanticgen classes

[kevinschaper]

We're currently using our very own special boutique biolink specific code generation software (biolink-model-pydantic), and we've been building up the linkml pydanticgen code to replace it. Pydanticgen may not have the same level of validation, which we'll make up for with adding linkml-validator to Koza.

• [x] Add a Makefile command to download biolink-model.yaml & generate the model (this can be borrowed from biolink-model-pydantic)
• [x] Change the import to point to the new model
• [x] Since we won't have a Predicate enumeration, replace Predicate. with 'biolink:'
• [ ] Write issues for using classes instead of strings for subjects and objects

[putmantime]

Look at how python gen gets the class URI.

[putmantime]

@kevinschaper please review biolink and linkml PR status for this.

[kevinschaper]

I'm working on cleaning up a couple of lingering biolink-model-pydantic imports that came in parallel to this

[putmantime]

Check Biolink to see if onset change has been made