Add heuristic plugin example

[Joao-Dionisio]

This supersedes #1245 . @abb-omidi , would you mind taking a look, please?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.27%. Comparing base (9547994) to head (f0db1a3).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1253      +/-   ##
==========================================
- Coverage   57.91%   57.27%   -0.64%     
==========================================
  Files          26       27       +1     
  Lines        5807     5915     +108     
==========================================
+ Hits         3363     3388      +25     
- Misses       2444     2527      +83     

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[Joao-Dionisio]

The disjunction example suffers from the bug mentioned in #1240, so an eventual fix would need to be merged first.