Fix connectivity definition in the adapter

[davidscn]

Closes #313

This was already anticipated in https://github.com/precice/openfoam-adapter/pull/297#discussion_r1273447571 std::map is sorted according to the key, which leads to wrong key-value pairs in the previous implementation, as the keys are resorted before we insert the (not resorted) values.

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

[MakisH]

Wouldn't it be simpler to replace the std::map with an std::unordered_map?

[davidscn]

Considering, which if these versions is more compact, I like the current version more. An unordered container would solve the problem, too, of course.