Feaure/dont use ug symlinks
The hack to have every sub-directory in the include path does no longer work for me. Time to get it fixed by making the include paths relative to dune-uggrid root directory.
- include paths relative to dune-uggrid root directory
- move header files below
dune/uggrid/
. - drop folder ug/ and compatibility with UG 3.13.
Supersedes !137 (closed) Fixes #13 (closed)