BGL: Add graph_traits for geometrycentral

[afabri]

Summary of Changes

Add a specialization of boost::graph_traits for `geometrycentral::surface::ManifoldSurfaceMesh". This should enable the usage of BGL algorithms which operate on non-mutable graphs.

This PR had as dependency PR 192 in the geometrycentral github project which is now merged into master.

Todo: work on out of the box properties. Add examples for the CGAL BGL extensions.

This code might be contributed to the geometrycentral project.

Release Management

• Affected package(s): BGL
• License and copyright ownership: LGPL, GeometryFactory