Implement mean occupation and get coherence functions

[labay11]

Checklist

Thank you for contributing to QuantumToolbox.jl! Please make sure you have finished the following tasks before opening the PR.

• [x] Please read Contributing to QuantumToolbox.jl.
• [x] Any code changes were done in a way that does not break public API.
• [x] Appropriate tests were added and tested locally by running: make test.
• [x] Any code changes should be julia formatted by running: make format.
• [ ] All documents (in docs/ folder) related to code changes were updated and able to build locally by running: make docs.
• [x] (If necessary) the CHANGELOG.md should be updated (regarding to the code changes) and built by running: make changelog.

Request for a review after you have completed all the tasks. If you have not finished them all, you can also open a Draft Pull Request to let the others know this on-going work.

Description

Includes functions mean_occupation and changes get_coherence.

Related issues or PRs

Implement #340

Additional context

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.33%. Comparing base (270d3f6) to head (29cae76).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #343      +/-   ##
==========================================
+ Coverage   93.14%   93.33%   +0.19%     
==========================================
  Files          41       41              
  Lines        2684     2762      +78     
==========================================
+ Hits         2500     2578      +78     
  Misses        184      184              

:umbrella: View full report in Codecov by Sentry.
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[ytdHuang]

Please follow the checklist above.

You can change this PR into draft PR before you think it is ready.

Otherwise it will trigger the CI pipeline everytime when you update the branch

[ytdHuang]

@labay11

To change a PR to draft PR, you can follow the steps here: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/changing-the-stage-of-a-pull-request

[albertomercurio]

You included also commits of other PRs. Could you just rebase and leaving only your commits? Tell me if you don't know how to do it, in the worst case I should be able to do it directly here.

[labay11]

Yes sorry, I didn't notice when I did the rebase.