Merge branch 'feature/example-cahn-hilliard' into 'master'

[example] cahn hilliard including full model

See merge request !3389

(cherry picked from commit d3264938)

42d4af8e [examples] Add Cahn-Hilliard model example
0bbdf76b [examples] Add description for cahn-hilliard model
04af2b71 [example] Test Cahn-Hilliard example
49f086ca [example] Make Cahn-Hilliard second example and expand description
c515a261 [example][cahnhilliard] Further expand and improve example description