[two-scale-heat-conduction] Fix parallel macro: Write data only for interior elements

[mathiskelm]

Coupling data was still written from the solution vector, not limited to interior elements / coupling points.

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.