DataSelection: If a user's hdf5 file has multiple datasets, ask which one to use before configuring the operator.
Working on #973
OpPixelClassification: checkConstraints() doesn't fail when a mask is provided to a lane that doesn't have raw data yet.
DataSelection: Once the user manually chooses an hdf5 dataset volume name, use it as a default for subsequent hdf5 files in the same role.
Along with c3c2456d129346e8d5a29afd2179823594fdf731, Fixes #973
Replaced all uses of OpSubRegion with OpSubRegion2.  Note: In the next pair of lazyflow/ilastik commits, the old OpSubRegion will be removed, and OpSubRegion2 will replace it (renamed as OpSubRegion).
Update to latest lazyflow change: OpSubRegion2 is renamed as OpSubRegion
shell: debug menu: auto-open workflow diagrams after they are saved.