operator.spin.sigmax interplay with fermionic spaces

[PhilipVinc]

I've seen some people in the wild use operator.spin.sigmaXYZ with fermionic Hilbert spaces to construct fermionic spin operators.

This does not crash, but the operators that are constructed are not fermionic and probably are nonsensical. We should warn or raise an error in those situations, and add some documentation about it.

Likewise, we could add fermionic spin operators to operator.fermion