This adds a couple features and bug fixes in pursuit of adding a feature to high-jack the callbacks of decimate to prevent (new) self-intersections to be made during edge collapses. To do this efficiently, I modified the AABB tree so that leaves could be dynamically updated (roughly following Eric Catto's notes).

Along the way I also found some bugs in the fast_find* functions (intersections with neighbors were ignored). I've added some routines to detect these intersections (approximately! using floating point ; i.e., not exactly like CGAL or with fancy igl::predicate*). The logic in these functions should be reusable so could benefit from abstracting over fancy types/predicates.

Added tests and tutorials.

I made this nasty "balloon inflation" mesh to create a high-res input that has no self-intersections but would cause a ton during decimation. The new feature successfully avoids all self-intersections.

Since it wraps existing callbacks it can be used with qslim metrics:

Would be cool to extend to a nested cages/progressive hulls type of constraint for cage / level of detail hierarchies. For now, you can at least go all the way down to super coarse with no self intersections:

Using the trivial decimate callbacks, you can kind of use this for self-intersection-free remeshing. Though it'd be much better to just use the dynamic AABB in the same way for a real remesher like Silvia's port of https://github.com/sgsellan/botsch-kobbelt-remesher-libigl