Update copying of ROI masks
Update copying of ROI masks

Instead of iteratively copying subranges of ROI masks, simply have
`h5py` copy the HDF5 Dataset (as it is now a self-contained Dataset) to
the new HDF5 file containing simply the ROI masks. This let's us cut
some more complicated code from data finalization.

nanshe-org/nanshe_workflow,DudLab/nanshe_workflow