Moved class MeshCoarsener to MeshLib/MeshEditing.
Moved Mesh2MeshPropertyInterpol to MeshEditing.
Moved ConvertRasterToMesh to MeshEditing.
Moved MeshGenerator to MeshEditing.
Moved some classes from folder MeshEditing to folder MeshGenerators.
Made CMake include paths relative.

Replace CMAKE_SOURCE_DIR with CMAKE_CURRENT_SOURCE_DIR and
CMAKE_BINARY_DIR with CMAKE_CURRENT_BINARY_DIR. This allows to use
OGS as a subdirectory in another project.
Removing nullptrs from element vector.
Add zlib to link libraries for CollapseMeshNodes.
Changing the read mesh functionality in order to read vtk meshes, too.
Removing unused nodes a bit more elegant.