better install directories
So far, XDMF installs its files into approximately the right places. Not everything is as it should be though: Library files are unconditionally installed into lib/
, Python files into lib/python/
(?!), and the CMake configuration into lib/
as well.
This PR fixes things, mostly by using CMake's own GNUInstallDirs. This should give a much better experience out of the box.