#109 ZTolerance and sign

[Mathieu Malaterre]

Double definition:
Line 30 of:
http://gdcm.svn.sourceforge.net/viewvc/gdcm/trunk/Source/MediaStorageAndFileFormat/gdcmIPPSorter.cxx?revision=6507&view=markup

and Line 246 of:
http://gdcm.svn.sourceforge.net/viewvc/gdcm/trunk/Source/MediaStorageAndFileFormat/gdcmImageHelper.cxx?revision=6507&view=markup

Also I'd recommend to change line 251 of gdcmImageHelper.cxx and 181
of gdcmIPPSorter.cxx to something which also test the sign of the
difference to be constant. More correctly the whole difference should
stay constant within the ZTolerance range (not just it's absolute
value). Because now also a dataset with alternating z-positions like
1,2,1,2,1,2,1,2 would be valid. Actually something like this might
happen, if some Dicom Stack mixes up temporal and spatial series.

[Mathieu Malaterre]

I believe this is fixed.