Add indexing and inner product to Statevector

[DavideFrr]

Summary

Make Statevector objects subcriptables to fix #4916
Add inner product method to Statevector objects to fix #4934

Details and comments

[javabster]

Hi @DavideFrr thanks for working on this! It looks like you've got an indentation issue that's causing the test failures and some linting (you can run tox -eblack for the linting). Could you also flesh out the PR description a bit more and add a release note please? For future reference if you put fixes #issue-number in the PR description Github will automatically link the issue to the PR which makes life easier for the reviewers 😄

[jakelishman]

Hi there, thanks for looking at this. I've left a few comments inline in the text - there's a few of them, but most of them are very minor style tweaks. There's a couple that bear a little bit of thought, though, about how we should be handling negative keys, and inner products with items that aren't statevectors.

[jakelishman]

The "suggestions" feature inline is usually for code reviewers as opposed to the person making the PR: we suggest, and then you commit them if you agree. If you want to change the code, it's your code, so you can just push the commits! In these cases, since you're suggesting stuff that was exactly in line with my comments, you can just go ahead and commit them directly, and I can review again.

[jakelishman]

Sorry it took me a little while to reply to this. The features look fine, and the tests of their working behaviour seem good, but please can we add some tests of the failure paths as well?

(When you've made a complete set of changes, if you press the "re-request review" button next to my name, it'll pop up my priority list.)

[jakelishman]

Thanks for sticking with this, it looks good now!