Fix Series.get_spatial_resolution() to return correct z-axis values

[jpcenteno80]

Hi, this is more of a question... I wanted to calculate voxel size from a dicom series and came across the get_spatial_resolution method from the Series object. I see that it returns pixel spacing together with slice thickness as a tuple. Since spatial_resolution is also used in determining the affine matrix for the series, shouldn't SliceThickness be replaced with "z spacing", calculated from ImagePositionPatient?