Relax requirement for orthogonal volumes.

This allows the reader to compute the slice spacing for gantry-tilted CT
series and to properly sort the slices.  It does not correct for the tilt,
that will be coming in a later commit.
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.