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
Sebastien Loriot
f6014dcc26