[doxygen] Exclude tests from doxygen?

There is a number of arguments for simply removing the tests and all files in the test directory from doxygen

• The is a large amount of classes and files that distract from the actual classes and files in the Dumux core

• The code doc of the tests is poorly structured and doesn't help much

• Unclear which information gain we get from inclusion

• Our doxygen takes very long to compile and is quite large. This would help reducing it a bit (all dumux source files in the dumux folder are 231,172 lines total, all source files (.hh,.cc) in the test folder are 90,691 lines total; using find . -name '*.hh' -o -name '*.cc' | xargs wc -l)

• Much of the doc is not very good (while that is a problem in itself and the doc comments in the tests should actually be of high quality too, this is in the current stage really distracting). Even if it would be better the other points remain.

I navigated through the test docs in the online class documentation a bit. I didn't find anything useful in the Test & Benchmarks section. Any other opinions and arguments for keeping it?