Improvements to plotting function for flow-over-heated-plate

[j-signorelli]

This PR updates and improves the plot-final-interface-temperature.py file:

• Previously, the file would only run properly when all solid solvers were run; now it works regardless of how many have been run
• When data was not found, the last file failed because data_id was not defined, and if not data_id would cause an error. This updated script correctly prints what files/data is/are not found in the search.
• General improvements (such as using enumerate, removing unnecessary retrieval of other data arrays, etc.)
• Because either SU2 or OpenFOAM may be used as the fluid-participant, the labels were changed as OpenFOAM-nutils to Fluid-nutils for example.

This PR also adds SU2 to the metadata.yaml file in flow-over-heated-plate.

No user-facing changes.

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.

[MakisH]

We somehow overlooked this for a while. I had a look now and directly modified a few things:

• Switched to reading .vtu files instead of .vtk, which is now the default. I had to adapt the reader for that.
• Removed the metadata.yml change. We should do this in a separate PR, and it is mainly meant for the system tests.

I tested this and it works:

The changes in the code also make sense.