Add voxel spacing support in 3D SLIC segmentation

Hi all from the EuroSciPy sprint!

This PR adds much-needed support for a `spacing` keyword argument to 3D SLIC segmentation.

This complicates the `sigma` calculation a little bit (we want to divide `sigma` by `spacing` along each dimension), so I added a `_sanitize_sigma` private function that ensures many kinds of input are allowed and "just work": scalar float, scalar int, list of float, list of int, arrays, etc.

Comments welcome!