Altermagnets

[lamoreel]

This pull request adds support for simulating d-wave altermagnets through a new Altermagnet class. The main difference with Antiferromagnet is the removal of the ferromagnetic exchange field in favor of an anisotropic field. In principle, the user is still able to take both fields into account, but that's their responsibility 😊.

For the time being, only open boundaries and 2D magnets are supported in the anisotropic exchange.

[DiegoDeGusem]

Nice and very clean! Just one general comment. The altermagnets are not included on the website. They should be added in api.rst.

[ilateur]

I would still like to leverage the object-oriented approach by making the Altermagnet a child class of the Antiferromagnet. It has exactly the same functionality, but expanded. Even if physically these are totally different materials, the implementation of shared aspects is identical. Though I must admit, the amout of copy-pasting and function-tweaking is less bad than I expected.

Other than that, everything looks clean to me with some very nice tests.

[ilateur]

Make sure to resolve the merge conflicts as well