NSitesAnalysis methods for squin.noise statements

[david-pl]

Noticed during #442 that there are no methods at all for squin's noise statements even though the number of sites is usually known.

[johnzl-777]

@david-pl I believe #342 covers some of this, I was (at the very least) supposed to support site analysis for depolarization although I wouldn't be surprised if there's other things that are unaccounted for

I don't mind implementing this considering it's been on my list for a while but I do see you've already assigned yourself to it

[david-pl]

Happy to re-assign 😄 if you end up not having time for that, just switch it back.