add OpenMesh_Mesh and CGAL_Mesh (OK with 0 error - 0 warning under Windows, Linux and Mac)

1) OpenMesh_Mesh :
 - OpenMesh_Mesh and OpenMesh_Traits : OK
 - OpenMesh_ReaderAdapter and OpenMesh_WriterAdapter : OK
 - OpenMesh_FilterAdapter : OK (but empty one for the moment...)

so -> see OpenMesh_FactoryMain test.

2) CGAL_Mesh :
 - CGAL_Mesh and CGAL_Traits : OK
 - no Reader and no Writer because we will use OpenMesh one, so it's normal
 - no Filter but I will add one very soon (triangulate one for example...)

3) Not in this PullRequest/Merge (but on my computer for the moment because not finished...) :
 - OpenMesh_Mesh to CGAL_Mesh converter
and reverse one (CGAL_Mesh to OpenMesh_Mesh)