Fix subcycling / checkpointing for two-scale-heat-conduction participant macro-dumux

[mathiskelm]

Checkpointing was mixed up with normal DuMux time-step advances, breaking the macro-dumux participant in the case of subcycling. Fixed checkpointing and subcycling and cleaned up the time loop.

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[IshaanDesai]

The results look as what we would expect them to. I would suggest leaving the smaller initial time step value for macro-dumux as it is now, which induces subcycling. That makes this tutorial a stronger test case. Changing the end time of the simulation is also not problematic in my opinion.