Performance and position count fixes for ND2

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/12477 and https://trac.openmicroscopy.org.uk/ome/ticket/12508.  I am intentionally leaving 12477 open though, as I think more investigation at the formats-common level is in order (but likely not appropriate for 5.0.x).

To test, verify that jobs are green, and that QA 9438 (corresponding to ticket 12508) has 4 positions and 4 channels.  Physical size X and Y should be present, as should exposure times.  Initialization times of large .nd2 files (e.g. ```nd2/richard```, ```nd2/guillaume```) should be slightly smaller with this PR included.