Add trixi participant to channel transport tutorial

[uekerman]

Open TODOs:

• [ ] Change geometry to include step
• [ ] Get proper cell coordinates (currently first-order average)
• [ ] Allow arbitrary polynomial degree (currently hacked to 2)

Joint work with @sloede

[MakisH]

This code is written for preCICE v2, but it should be straight-forward to upgrade it to preCICE v3: https://precice.org/couple-your-code-porting-v2-3.html