ignore B dimension

According to CZI doc, B dimension is Block index in segmented
experiments. CZI no longer use it, however, older files may still have
B dimension, for example, the Young_mouse.czi from
https://zenodo.org/records/10577621

 . Ignore B dimension enables
OpenSlide read such file.

Signed-off-by: Wei Chen <chenw1@uthscsa.edu>