Don't remove `Atoms.specie`

[pfebrer]

When running code with the main branch, I get:

(dep:0: SislDeprecation: specie is deprecated, use species instead. [>=0.15] [removed in 0.16])

I think it is a bad idea to make specie not work for sisl>=0.16. It will break lots of codes unnecesarily. We could simply not document it or document it with a big warning stating that it is not the desired way of using it. In general I think we should start to care more about backward compatibility, now that there is a substantial user base.