Feature/mmgs nm

[Algiane]

⚠️ NOT TO MERGE RIGHT NOW ⚠️

🚧 Implementation in progress 🚧

This branch is a preliminary work on a full support of non-manifold geometries in mmgs. I current release of Mmg, building of adjacency relationships doesn't allow to travel triangles in the shell of a non manifold edge. Thus:

• nodes along non-manifold edges are marqued as required;
• insertion is authorized during non-conformal step;
• insertion, collapses and node relocations are forbidden in other parts of the code.

This branch proposes to build adjacencies along non-manifold edges using successive pointer toward next triangle of the shell (non-symmetric relationships between triangles) and starts to update some of the Mmg operators (node insertion, node collapses, ball computations..) to this new storage.

Debug was in progress when the work was abandonned