Mesh connectivity for preCICE v3

[MakisH]

With preCICE v2, the adapter does:

• set mesh vertices
• ask OpenFOAM to compute triangles
• give the triangle nodes coordinates to preCICE and ask for vertex IDs
• use the vertex IDs to define triangles

With this PR (and preCICE v3), the adapter does:

• create a std::map from vertices (std::tuple of double-triplets (coordinates)) to vertex IDs, initializing the vertex IDs to -1.
• set mesh vertices
• Assign vertex IDs (values) to the point-triplets (keys)
• ask OpenFOAM to compute triangles
• Parse the map the define triangles using the map to get IDs

I ran the flow-over-heated-plate-nearest-projection tutorial and the results look similar. Comparing all VTK files with fieldcompare, I get some successes, some failures where the difference is on the last digit, and some failures where just fieldcompare could not open the files.

co-authored by @fsimonis

Depends on #285

Closes #258.

TODO list:

• I updated the documentation in docs/ -> N/A
• I added a changelog entry in changelog-entries/ (create directory if missing)

[davidscn]

Functionality looks good (haven't tested it). Only minor comments below.

[MakisH]

After #285 is merged, I would rebase this branch and merge it next.
I think it is ready.

[MakisH]

The CI is expected to fail because it is using preCICE v2.5.0. I will update this in a separate PR.