Doxgen for 3.2

Todos:

• empty doxyerr.log
• doxygen not aborting, for this see doxygen.log
• correct display of the equations, dependency-plots/inheritance-plots (there is a plot called aXY+1 included in the file aXY.html) -> OK on https://dumux.org/doxygen/master/index.html
• correct linking of the sites (see sanitizelinks.sh for previous fixes)
• correct assignement to submenus
• does the documentation/text of the submenus still make sense/can it be improved?
• does work on the dumux website as well?
• 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