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.
The preference is to follow VTK's convention for anisotropic
rectangularly sampled data, using the keyword `spacing` rather
than `sampling`. This change converts the last remaining use of
`sampling` in scikit-image to `spacing`.
Josh Warner (Mac)