Add generative models for crystal structure generation

To make it easy for people to get started with materials science, there's no better intro than to let them create their very own materials to study.

These kind of models are also especially useful for researchers working on material discovery challenges, like the teams #permanent-magnets, #thermoelectrics, and #superconductors.

So far, Ouro users have access to:

• MatterGen from Microsoft

• CrystaLLM

• Chemeleon

• Custom approach I made wrapping PyXtal

Let's expand this set of models!