Zeiss CZI: adjust for incorrectly recorded extra dimensions

In some cases, the extra dimensions (e.g. 'positions') will be set to > 1
when only one e.g. position was actually acquired.  This commit corrects
for that when assigning plane indices, which has the effect of fixing
missing stage positions and timestamps.

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/11750