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.

Everything in our .hh files is defined within the namespace `Dumux`.
There were numerous occurrences of `Dumux::` qualifiers that were
unnecessary. Remove those occurrences. Necessary qualifications
remain, such as `typedef Dumux::H2O<Scalar> H2O;`.

Clang warned about && and || without parenthesis which might
confuse readers

Updated the tutorial and its description in the handbook to use implicit instead of coupled and sequential instead of decoupled.