hdf5: reinstate DyND coercion on __iter__ and chunks

We avoided this previously because DyND didn't handle NumPy strings well
Now it does.

This allows us to view data stored in HDF5 under different dshapes.
Data is coerced through DyND on its way out.
Merge pull request #370 from mrocklin/hdf5-coercion

hdf5: reinstate DyND coercion on __iter__ and chunks