Doxygen for 3.5

Todos:

• empty doxyerr.log as much as possible
• Anything interesting to fix in doxygen.log? There is a warning "warning: Tag 'COLS_IN_ALPHA_INDEX' at line 114 of file 'Doxyfile' has become obsolete." However, this concerns dune-common/doc/doxygen/Doxystyle, not dumux.
• does doc/doxygen/modules.txt match the folder structure?
• Check for missing ingroup
• Check for wrong ingroup
• check correct display of the equations
• correct linking of the sites (see sanitizelinks.sh for previous fixes)
• does work on the dumux website as well?
• under Namespaces - Namespace List spot some undocumented classes
• does the documentation make sense? spot mistakenly made copy& paste?

For possible reference of a future doxygen dude: The documentation is built by running make doc in the dumux build folder. One way to switch doxygen versions is to:

• download the doxygen git repository from https://github.com/doxygen/doxygen.git
• checkout the branch you want, e.g. Release_1_8_17
• the first time do: mkdir build, cd build, cmake -G "Unix Makefiles" ..
• always do in the doxygen build directory: make, sudo make install
• the first time do: export PATH=/usr/local/bin/doxygen:$PATH
• delete the dumux build folder and rerun dunecontrol for dumux ./dune-common/bin/dunecontrol --opts=./dumux/cmake.opts --only=dumux all

After Changes in scr/doxygen.cpp delete the build folder an recreate it.

Related MRs: !3027 (merged), !3029 (merged)