Change SingleSiteOperator with MultiSiteOperator

[albertomercurio]

Checklist

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Description

Here I change the SingleSiteOperator with a more general one MultiSiteOperator, allowing to generate an arbitrary product operator of a multipartite system.

Related issues or PRs

This PR fixes #322.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.67%. Comparing base (ab66af5) to head (e2520ae).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #324      +/-   ##
==========================================
- Coverage   93.07%   92.67%   -0.40%     
==========================================
  Files          36       36              
  Lines        2585     2595      +10     
==========================================
- Hits         2406     2405       -1     
- Misses        179      190      +11     

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