Cleanup/doxygen for 3.0

Merged Simon Emmert requested to merge cleanup/doxygen-for-3.0 into master

This is the second part where we eliminate most errors and warnings produced from doxygen.

Be aware that I included one commit that changes the forward declaration of the 2p2c sequential fvpressure.hh etc. They were producing a lot of warnings because the only the types but not the arguments were declared and therefor only the types did show up in Doxygen. I am not sure if this is the desired solution, but I could not think of anything else. For me it looks like it does not change anything. Doxygen seems to have some issues with forward declaration in general. So if this is not the desired solution we can go back and just live with the warnings until we change the sequential things anyways.

@timok, @becker, @bernd what do you think? Can we simply extend these declarations like I did or can this be a problem?

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