Merge branch 'origin/fix/tutorial-exercise1f' into 'master'

[fix/tutorial-exercise1f] Exercise 1f makes more sense now

The changing constitutive relations task in exercise 1 previously was about
exchanging the EffToAbsLaw with a LinearMaterial law. It makes more sense to
replace the RegularizedBrooksCorey.
The description in the handbook was adapted accordingly.

See merge request !248