CMake Install pdelab.hh and backend/istl/matrixfree

Hi all,

at the moment, the pdelab.hh file and the files in backend/istl/matrixfree are not installed. In order to install these files, I propose to add the file dune/CMakeLists.txt with the following content.

add_subdirectory(pdelab)
install(FILES pdelab.hh DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/)

The root-level CMakeLists.txt has to be changed accordingly, i.e.

add_subdirectory(dune)

instead of

add_subdirectory(dune/pdelab)

And the matrixfree subdirectory has to be added to backend/istl/CMakeLists.txt.

Best, Christian