Also refactors to reduce copies into numpy arrays,
and use vecs of vecs less.

So that it is actually signed.
Also some documentation.

- 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

switch to pseudonormal containment check is now n_rays=0 rather than
None, due to PyO3/pyo3#3735