8-bit hdf5 images are converted wrongly on writing for elastix

[K-Meech]

At the moment, hdf5 images are read with 16-bit imageloaders in bigdataviewer - see: bigdataviewer/bigdataviewer-core#128

This gives weird results, as I then try and convert from 16-bit to 8-bit to write for elastix and end up writing images to a strange range. Would be easiest to fix upstream.
Use n5 for now.