Export slice position sort

[dillonwilliams]

_sort_by_slice_position in combine_slices.py would be useful to dicom_numpy callers so that instance metadata (e.g. SliceThickness) can be associated with the volume generated from combine_slices.

[yujanshrestha]

@dillonwilliams can we just call combine_slices once per DICOM file? Then you do not have to rely on the ordering out of combine_slices. It will also be a good way of handling volumes with non-orthogonal and missing slices. It will also give you one transformation matrix per slice.