Add Hertel-Mehlhorn algorithm

[EmbersArc]

Closes #62.

Again, I'm not sure how to make this accessible to the user. I've added it as a method to TriMesh for now, but maybe adding it as CompoundShape::from_trimesh() makes more sense.

[sebcrozet]

Thank you for this PR, this is fantastic!

I suggest the following, to keep it coherent with some other parts of the crate (like convex_hull2):

• Rename the hertel_mehlhorn function to hertel_mehlhorn_idx.
• Add a new hertel_mehlhorn function to the hertel_mehlhorn.rs file. This new function returns a Vec<Vec<Point<Real>> (it just calls hertel_mehlhorn_idx and replaces all the indices by the corresponding points).
• Re-export both hertel_mehlhorn and hertel_mehlhorn_idx from the transformation_module.
• Like you suggested, I think it is better to add a CompoundShape::from_trimesh() instead of adding the method to TriMesh. I’m not sure about the naming though. from_trimesh may suggest that we are converting each triangle to it’s own shape in the compound. Maybe we should call it something like CompoundShape::decompose_trimesh.

[EmbersArc]

Thanks for the review @sebcrozet! I've applied the suggestions in 7d9026b.

[sebcrozet]

Perfect, thank you!