[properties][cleanup] deprecate property GridView and replace by GridGeometry::GridView or Grid::LeafGridView

Reflect the change introduced by commit 83fe871a to the Dumux master.
Prevent deprecation warnings.

[course][exercises][naming] adapted towards the general naming conventions, unified naming of files and executables for the course

removed TypeTag Endungen

changed captions Task 2, Task 3 and Task 4 to appear bold in the readme

changed to
box.hh
cctpfa.hh
ccmpfa.hh