Mesh 3 - avoid tuples in internal code

[janetournois]

Summary of Changes

This PR introduces new structs to replace std::tuple's in internal code of Mesh_3, and in particular avoid the use of std::get<i>(tuple) that can be hardly readable, compared to the use of explicit member names like my_tuple_struct.dimension

Release Management

• Affected package(s): Mesh_3
• License and copyright ownership: unchanged

[janetournois]

@lrineau what do you think about this internal change?

[sloriot]

Several errors detected in CGAL-6.0-Ic-271 In Mesh_3, Poisson et Tet remeshing

[sloriot]

Successfully tested in CGAL-6.0-Ic-277

[sloriot]

Successfully tested in CGAL-6.1-Ic-3