Add `voxel_size` argument to all functions which return voxels sizes

[jgostick]

Currently functions like porosity_profile and satn_profile return a results object with position as an attribute, but these positions are in units of voxels. Might it be a good idea to allow uses to specify voxel size, and if so, to scale the position values accordingly?

[ma-sadeghi]

I think it’s better to keep the results in voxel units. This is more extensible, especially for computationally expensive functions like network extraction. If someone accidentally uses the wrong voxel size, they’d have to redo all the calculations—unless they’re willing to risk rescaling themselves, which isn’t always reliable.