Add concurrency support for effective_hamiltonian in labs

[soranjh]

Context:
Implements concurrency in effective_hamiltonian to support running parallel jobs.

Description of the Change:

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.69%. Comparing base (4d684c6) to head (d4251b5).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8081   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files         548      548           
  Lines       56845    56845           
=======================================
  Hits        56672    56672           
  Misses        173      173           

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[willjmax]

Just two minor changes needed.

[obliviateandsurrender]

Don't forget to add a changelog entry. 🚂

[willjmax]

Looks good 🚀