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

EnableGridGeometryCache

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

changed to
box.hh
cctpfa.hh
ccmpfa.hh

updated README

update code main file and problem file