[tracer][reference] implements an outflow boundary for tracer problem
What this MR does / why does DuMux need it: The tracer problem had neumann-noflow boundaries so far. We now implement an outflow BC on the top so that the tracer is transported out of the domain and does not accumulate anymore.
The reference solution is adapted.
- update examples folder accordingly
Depends on !1808 (merged)
Which issue this MR fixes: fixes #784 (closed)
Special notes for your reviewer: Only tested the tracer tests so far.