Support for prestack offset-volumes

The data from Volve in the ST0202vsST10010_4D/PreMig_data/Offset_gathers folder (ST0202ZDC12_PZ_PSDM_INPUT_OFFGRPS_T.ST0202-ST10010-PRESTACK-RECEIVER-SEGY.NORWAY.4230.23031.1613.1.sgy) cannot currently be imported properly since it should result in a 4D Sample/Trace(offset)/Crossline/Inline volume with dimension-group 0/2/3 bricks, similar to how crossline-sorted volumes have the same axis but dimension-group 0/1/3 bricks instead of the 'normal' dimension-group 0/1/2 bricks that are produced when the volume is inline-sorted. It will require some modifications to how the file-info is created since the 'fold' is basically separate inlines for each offset, so we still want to pretend that inline is the primary key when creating the file-info.