Volume coupling and nearest projection mapping do not integrate well and are untested

[BenjaminRodenberg]

The issue https://github.com/precice/tutorials/issues/374 basically originates from a bad integration of volume coupling into the overall code and a lack of testing. I decided to comment out the corresponding code to keep the existing surface coupling features working.

Todos

• [ ] We need some tests that ensure that volume coupling and surface coupling with nearest projection mapping don't conflict
• [ ] We need a proper integration of both features
• [ ] Add channel-transport-reaction as a pure FEniCS case to our tests

[BenjaminRodenberg]

I changed the milestone of this issue to v2.x.x since #163 is part of that milestone and I don't think we can currently prioritize this issue for the next release.