SDF internals, robust containment checks, benches

- private method _sdf_intersections, which casts rays and returns the
  distance to a mesh and the dot product of the ray and the face normal
- containment checks now may use signed distance, which is slower in
  real-world meshes but more robust
- ray-based containment checks now may use consensus of a number of rays
- different containment check strategies are benchmarked

.gitignore

@@ -102,3 +102,5 @@ ENV/
 .mypy_cache/
 
 .benchmarks/
+
+tmp/