Add a heat flux wrapper

[GardevoirX]

Closes #104

This is still in a very early stage and subjected to change.

TODO:

• Interface for getting the heat flux from the model
• Checks for the applicability of unfolding the system
• Tests
• Documentations
• Remove the dependency on vesin.metatomic and rebuild neighbor list from the un-unfolded system

Contributor (creator of pull-request) checklist

• Tests updated (for new features and bugfixes)?
• Documentation updated (for new features)?
• Issue referenced (for PRs that solve an issue)?

Reviewer checklist

• CHANGELOG updated with public API or any other important changes?