Add mixed BC heat conduction test cases

[vidulejs]

Adapted from the partitioned-heat-conduction tutorial case. The only real changes are the interface boundary conditions (0/T), system/preciceDict, and precice-config.xml. The remaining files were copied from the base tutorial.

TODO: Summarize and motivate the changes, link to issues, remove the checklist entries that are not relevant.

Checklist

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.

For new tutorials or tutorial cases:

• I followed the tutorial folder structure
• I added/updated the tutorial README.md
• I added/updated the tutorial metadata.yaml
• I added tests in tools/tests/tests.yaml
• I submitted a pull request to the website with:
  • An entry in _config.yaml
  • A sidebar entry
  • An entry in the tutorials overview

[MakisH]

These tutorials were mostly meant as test cases for features of the OpenFOAM adapter (and I asked @vidulejs to quickly open this PR to have them available).

Since these are variants of existing tutorials that mainly relate to the OpenFOAM adapter configuration, and since that configuration will eventually become the standard, I prefer to not merge this PR, but keep it as a test case. I might reopen this PR in the future.

We have now integrated these tutorials as test cases sourced directly from your fork in #732, included in the nightly release test suite. This now also serves as a test case for the "external source" feature of OpenFOAM.

I am closing this PR, but please do not delete the fork and the branch.