Handle integer treated variable in RegressionDiscontinuity experiment class

[drbenvincent]

• Closes Better handling of treated input in RegressionDiscontinuity #440
• Now accepts int types for the treatment variable.

---

📚 Documentation preview 📚: https://causalpy--506.org.readthedocs.build/en/506/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.17%. Comparing base (6566232) to head (e933420).
⚠️ Report is 42 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #506      +/-   ##
==========================================
+ Coverage   95.13%   95.17%   +0.04%     
==========================================
  Files          28       28              
  Lines        2384     2406      +22     
==========================================
+ Hits         2268     2290      +22     
  Misses        116      116              

☔ 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.

[JakePiekarski314]

Looks good to me!