Add sophistication to slice spacing computation.
Add sophistication to slice spacing computation.

Compute the slice spacing to be the average slice spacing after culling
outliers.  This makes the spacing computation robust when a series is
missing slices.  Unfortunately, everything past a missing slice is simply
shifted by one slice, which is not the right thing to do... in a later
commit, the gaps should be filled with black slices or perhaps with
interpolated slices.

dgobbi/vtk-dicom,dgobbi/vtk-dicom,dgobbi/vtk-dicom,hendradarwin/vtk-dicom,hendradarwin/vtk-dicom,hendradarwin/vtk-dicom