Add functions to compute the kernel of a mesh ## Release Management * Affected package(s): `PMP`, `BGL` * Issue(s) solved (if any): * Feature/Small Feature (if any): [Mesh_Kernel](https://cgalwiki.geometryfactory.com/CGAL/Members/wiki/Features/Small_Features/Mesh_Kernel) * Link to compiled documentation (obligatory for small feature) [*wrong link name to be changed*]() * License and copyright ownership: GF