ENH: Simplify marching_cubes for maintainability.

This does not change execution time, but the entire algorithm
is much simpler to read and understand without the branching
Cython code paths for anisotropic inputs. Memory usage decreases
a minor amount.
Merge pull request #1115 from JDWarner/enh_marching_cubes

ENH: Simplify `marching_cubes` for maintainability