Add a way to pass input normal to faces

[sloriot]

Avoid computing them when they are known.

• Normals are only used for estimating the angle between neighbor faces. PCA is used to estimate the plane.
• sorting should be reusing the triangulation of faces that is already done in regions (pass the internal map to sort() for exemple). PCA is also used here to estimate the flatness score of a face in its neighborhood
• About sorting: I'm not sure it wouldn't be a better idea to simply sort using the area of the faces and ignore flatness of the neighborhood.