Merged requested to merge feature/example-cahn-hilliard into master
Similar intent as example of !3380 (merged) implementing a nonlinear model with two equations.
Parallel should work with e.g.
DUMUX_NUM_THREADS=1 mpirun -np 4 ./example_cahn_hilliard (multi-threaded assembly will not help much because the main time is the solver, could try different solver with !3386 (merged) or the solver factory).
Add example description and doc generator