h5pyd dataset.chunks not compatible with h5py

[jreadey]

dataset.chunks can return a dict object for h5pyd datasets that are linked to HDF5 files. This could cause some compatibility problems when converting scripts to use h5pyd.

[jreadey]

With the above checkin, dset.chunks always returns None or a tuple.
Note: if you need the information formerly returned by dset.chunks as a dict, you can get the chunk layout using dset.id.layout. E.g:

{'class': 'H5D_CHUNKED_REF_INDIRECT',
'dims': [1, 500000, 1],
'chunk_table': 'd-889b0036-04f59506-3f58-c1ea8e-320902'}

[jreadey]

This is merged into master.