Fix Z and T size calculation for InCell 6000 data

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/12416.  There is one .xdce file attached to the ticket which needs to be verified - there should be 96 wells with 16 fields per well and 1 Z section/timepoint.  Without this change, multiple Z sections were detected.  This is the only InCell 6000 dataset we have; I would not expect other datasets to be affected (so it's sufficient to verify that repository tests pass).

See as well https://www.openmicroscopy.org/community/viewtopic.php?f=4&t=7545#p14250.