Use GRIDSDIR instead of absoulte path; fix GRIDSDIR path in CMake.

This fixes out-of-source builds for several tests.
[CMake] Add flags for ALUGrid, UG, ParMETIS for all tests.

This follows the way it's done with autotools.
[CMake] Add tests for dune/pdelab/test/fem.

The buildsystem only auto-adds tests that are located in a
folder called "test". Thus the tests have to be added
manually.
[Documentation] change documentation in convectiondiffusiondg
[FEM] Use QkLocalFiniteElement in Q22DLocalFiniteElementMap to avoid breakage on dune master