Implement Pauli sorting

[vaferreiQMT]

Howdy! This is the first of two PRs on transformations of PauliStrSums mainly focusing on improving the accuracy of Trotter methods. This PR specifically introduces methods to sort the terms within a PauliStrSum.

Main changes:

• Core: added methods to calculate and apply permutations to sort a PauliStrSum.
• API: Added methods to sort lexicographically and by magnitude (along with tests).
• Documentation: added a Setters section for Pauli docs, for the sorting methods and future methods that transform Pauli strings.

Key details:

• Since the sorting logic is only expected to be used outside hot loops, I have tried to keep the permutation logic as simple as possible and not reinvent the wheel, e.g. using std::sort rather than some custom implementation.
• When applying the permutations we require both some temporary storage to construct and invert the permutation, however since the number of terms in a PauliStrSum generally only scales polynomially, this shouldn't be an issue (e.g. even for extremely large chemical systems this additional memory would be negligible)

I think that about covers it. As always happy to tweak, move or update stuff if anything jumps out at you.

Cheers!