Thank you for the example, I checked it out and it seems that it can be easily adapted to my problem. I have couple more questions concerning the surface integrals and custom solvers in a mixed-dimensional branch.
Concerning solvers, is it possible to use a custom made nonlinear solvers, such as one given here and is it possible to use PETSc SNES. I noticed a comment in the demo file stating that SNES is not available yet.
How is marking of the boundaries done in the mixed-dimensional branch, is it the same as in the old version, by using MeshFunction()?