Non manifold surface does not lead to an error anymore : warning instead

[corentin-prigent]

This PR allows to return a mesh if the mesh adaptation stage leads to attach the discretized level-set surface to itself or to the bounding-box in a non-manifold way, i.e. the level-set surface touches itself or the box in one vertex only (2D, surface and 3D meshes) or in one edge only (3D meshes). See attached pictures for an illustration of what is considered as manifold level-set surfaces (top images) and as a non-manifold one (bottom images).

Manifold level-set surfaces :

Non-manifold level-set surfaces :

As it is not an expected behaviour (if we start the mesh adaptation stage with a manifold level-set surface, we should end with a similar topology), an error message and the MMG5_STRONGFAILURE return value was previoulsy returned without saving the mesh. As the mesh has been entirely remeshed and the generated mesh remains conformal and usable, now, we only raise a warning and the mesh is returned to the user (so the user can decide to use it or not).

Note that we still want to raise an error and to refuse to remesh the mesh if the level-set is non-manifold at the end of the stage of level-set discretization (before mesh adaptation). The reason is that we can consider such kind of configuration as a transitional step between the situation where the level-set is not attached to itself or to the bounding-box and where the situation where it has merged and for structural optimization for example, it is preferable to avoid remeshing during this transition.

[Algiane]

Thanks for this PR.

[Algiane]

It's all good, thanks!