Initial CMake support.
Reflect dropped template argument in VirtualOrderingBase.
Fix headercheck for most headers.

powercouplinggridfunctionspace.hh remains failing.
Fix make headercheck, especially these in test/.
Use new constructor for YaspGrid.
Reduce numer of errors with Dune 2.3 / PDELab 2.0.

Especially replace ISTLVectorBackend<1> by ISTLVectorBackend<> in all tests.
Adapt testassembler to new interface.
Remove unused local typedefs.