Validation with Pydantic

[mirkolenz]

It is already possible to manually use Pydantic with CBRkit (i.e., by not using our built-in loaders and using Pydantic models instead), but we currently do not have documentation on how to do it. We should add a corresponding section to the docstring in the file cbrkit.loaders.__init__.py.

Additionally, we should explore how to directly integrate Pydantic with the built-in loaders so that the user only needs to pass the Pydantic model in order to have the entire case base validated.

[mirkolenz]

Implemented in #87