Python marching cubes improvements

Summary: Overhaul of marching_cubes_naive for better performance and to avoid relying on unstable hashing. In particular, instead of hashing vertex positions, we index each interpolated vertex with its corresponding edge in the 3d grid.

Reviewed By: kjchalup

Differential Revision: D39419642

fbshipit-source-id: b5fede3525c545d1d374198928dfb216262f0ec0

Author: Jiali Duan