ring-diff arrays are not set appropriately for blocksoncylindrical

[KrisThielemans]

See comments in #1006, e.g. https://github.com/UCL/STIR/issues/1006#issuecomment-1162064887_

[markus-jehl]

This can be closed, right?

[KrisThielemans]

I think so, but cannot check at the moment. @danieldeidda do you remember?

[danieldeidda]

not sure, the comments say that this was going to be done in another PR but there was no error related to this as that change was reverted

[KrisThielemans]

This was about the initialisation of segment_axial_pos_to_ring1_plus_ring2, in particular https://github.com/UCL/STIR/blob/4142d5969e4320e32ed9dbe463b0fd50f36fb3f6/src/buildblock/ProjDataInfoCylindrical.cxx#L291-L293. The use of m_offset is unsafe for a blocks/generic scanner (due to irregular sampling). I believe that the end result of the calculation would still be fine, but this would need to be checked.

For instance for span=1 (which is currently used for blocks/generic), get_ring_pair_for_segment_axial_pos_num(ring1, ring2, seg, axial_pos) can be tested as seg = ring1 - ring2 (or maybe reversed) and axial_pos = min(ring1, ring2).

[markus-jehl]

This was tested for the NeuroLF scanner geometry, and the function indeed returns the correct results: for seg=5 and axial_pos=5 it returns 5 and 10, and for seg=2 and axial_pos=14 it returns 14 and 16.