Streamlining spin/index arguments on `read_grid` methods

[zerothi]

          > > It seems to me that line 35 is then wrong, no?

Oh, I see. Yes, indeed index=1 would give the spin density and not the down-component as currently stated. By the way, why two keywords index/spin for the same thing? Wouldn't it not be better to have just one?

Yes, currently this is a left-over from the siesta output. A think we could investigate is whether index should be used for explicit retrieval (no manipulation), and spin="x" could be used for explicit spin-configuration extraction, or spin=Spin.X.

Probably that should also be stream-lined.

Originally posted by @zerothi in https://github.com/zerothi/sisl/issues/754#issuecomment-2069320431