Allow rescaling SNOW based on voxel size

[ma-sadeghi]

Currently, voxel size is passed as an argument to SNOW, meaning that once the network is extracted, it's not possible to easily rescale the output based on a different voxel size. It'd be handy if there was an option to rescale the network afterwards. This is specially useful for large extractions that might take days to complete.

[jgostick]

I think this is possible by hand, but is tedious and error prone. A function to do it make sense.