- add ring_difference and axial_compression in IMG struct,
  and read/write it in ECAT7 3D sinograms.
- check why storage_order (3D scan) is changed from 0 to 1, maybe
  should be included in IMG struct too?
- Remove local unit definitions and use CUNIT_ and TUNIT_ definitions,
  currently in libtpccurveio, but later only in libtpcmisc. 
