A0-1672: units returned by run_session

[fixxxedpoint]

A separate method in member run_session_for_units that allows one to track finalization of batches of units.

[github-actions[bot]]

Please make sure the following happened

• [ ] Appropriate tests created
• [ ] Infrastructure updated accordingly
• [ ] Updated existing documentation
• [ ] New documentation created
• [ ] Version bumped if breaking changes

[joschisan]

@timorleph the advanced interface works for us just as well as far as I can tell.