Fix connectivity definition in the adapter

[davidscn]

Closes #313

This was already anticipated in #297 (comment)
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.

[MakisH]

The changes make sense, I have not tested anything. I also like this implementation better, as it moves the creation of the map after the setMeshVertices call.

We could also update the example on https://precice.org/website-testing/couple-your-code-defining-mesh-connectivity.html to use an unordered_map, as I guess more people will overlook this detail in their implementations.