feat: add default_target_function

[ReubenJ]

Add a function that creates a default target function for an entity of a QN as described in Eq. 3 from the QN paper.

Checklist

• I am following the contributing guidelines
• Tests are passing
• Lint workflow is passing
• Docs were updated and workflow is passing

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.89%. Comparing base (8ca5401) to head (2be692b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   97.70%   97.89%   +0.19%     
==========================================
  Files           2        2              
  Lines         174      190      +16     
==========================================
+ Hits          170      186      +16     
  Misses          4        4              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.