Zeiss CZI: prepend 0s so that image names are all the same length

See QA 10335 and
https://www.openmicroscopy.org/community/viewtopic.php?f=13&t=7681.  With this change, the index in all image names should be the same length (i.e. ```'#01```, ```#02```, ..., ```#20``` instead of ```#1```, ```#2```, ...).