test: add a validation test for binary mixtures

[V-Francois]

Summary

This PR introduces a validation test, which aims at testing the physical results of the code as a whole, rather than independent bits of the code (usually handled by unit tests).

This tests runs a binary mixture of LJ particles, at different conditions, and compares results to published data.
It validates that the overall MC algorithm, the displacement move, and the LJ model are properly implemented.

[V-Francois]

@romainljsimon , @leonardogalliano : I mostly did that as a tutorial for myself to learn about the codebase in a "hands-on" fashion.
Feel free to dismiss if you think it shouldn't be part of the main repo.

I personally like to include these validation tests in repos, as it gives more thorough examples to potential users, and can serve as non regression tests/long tests when doing major changes.

[codecov]

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[romainljsimon]

@V-Francois I think it's a great idea but I'm not sure if you want or not this to be part of CI/CD

[V-Francois]

@V-Francois I think it's a great idea but I'm not sure if you want or not this to be part of CI/CD

No. It takes a lot of time to run, and currently there's not actual check, I just did a visual look at the plot.