Modernize Models & Validation with Pydantic

[ax3l]

It was suggested today by @roussel-ryan to use a more formal schema to abstract PICMI inputs and extensions and compatibility with different codes.

Refs.:

• https://docs.pydantic.dev/usage/models/
• https://docs.pydantic.dev/usage/validators/

[roussel-ryan]

Reference for use in xopt https://github.com/ChristopherMayes/Xopt/blob/main/xopt/generators/bayesian/options.py

[ax3l]

This is similar to #63 (more) / #71 (less) but with clear paths to validate and being able to export data models for post-processing.

[BrianMarre]

I agree with @ax3l abstracting PICMI further from the codes would be a good idea
and testing for correct configuration on the picmi side would also be helpfull.