move DOMHEX code to contrib/domhex + make it optional

CMakeLists.txt

Common/GmshConfig.h.in

Mesh/BackgroundMesh3D.h

Mesh/CMakeLists.txt

Mesh/Generator.cpp

Mesh/meshGFace.cpp

Mesh/meshGFaceDelaunayInsertion.cpp

Mesh/meshGRegion.cpp

Mesh/meshGRegionLocalMeshMod.cpp

Mesh/meshGRegionLocalMeshMod.h

contrib/domhex/ThinLayer.cpp

contrib/domhex/ThinLayer.h

contrib/domhex/directions3D.cpp

contrib/domhex/directions3D.h

contrib/domhex/meshGFaceQuadrilateralize.cpp

contrib/domhex/meshGFaceQuadrilateralize.h

contrib/domhex/mwis.hpp

contrib/domhex/pointInsertion.cpp

contrib/domhex/pointInsertion.h

contrib/domhex/pointInsertionRTreeTools.cpp