Rename PauliString to PauliVec

[Aerylia]

PauliString strictly refers to operators acting on qubits, but it is used to store the letters per qubit for performance reasons.
Thus the datatype should reflect this: it is a vector of Paulis.

[y-richie-y]

+1. What if we called them PauliColumns even?

[Ectras]

Aren't PauliPolynomials stored row-wise (and using a PauliString for each row)? In that case, PauliVec would be better