volume-coupled-flow: add y and z components in source term

[MakisH]

Closes #542.

At the moment, the source term only considers the x-component of the read velocity in the volume coupling, even though we do exchange 2D velocities. Even though this example is completely artifical, reading only the x component seems to be unmotivated, and it already triggered a related question by @fsimonis when using this tutorial as a starting point.

Even though we exchange 2D data in preCICE, this is copying between OpenFOAM objects, which are both 3D structures.

@tirgendetwas do I miss something here, or was this simply something that slipped the review?

Results currently:

Results with the change:

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]

Ready to review (maybe @davidscn?).
This tutorial remains a very arbitrary/constructed or even plain wrong example just to demonstrate the concept (we knew that since the design phase), but I think that this PR still fixes a bug.

[davidscn]

I think the changes here require rather a conceptual review than a technical one and I am not familiar with the tutorial, what it is supposed to showcase or what the coupling is supposed to comprise. So, let me remove my review..

[uekerman]

The change makes sense to me, the tutorial not completely 😁