improvements to the CMake build system

- it now can generate and install the API documentation ('make install-html')
- the information of the dune.module file is now used much more extensively
- presence of the valgrind headers is tested
- tests which cannot be run for a configuration will now appear as
  'skipped' in CDash
- adding tests does not require modifying the toplevel CMakeList.txt anymore
- introduction of the 'EwomsAddTest' cmake macro which removes quite a bit of the
  copy-and-paste from the CMakeLists.txt.
- various small loose ends have been fixed
add reference solution for newly added test 'test_dec2p2c'
cube grid creator: fix a newly appearing spurious GCC warning if optimizations are enabled
also avoid warning if mpihelper.hh is included in a .cc file