data: discover on h5py groups and files
h5py: Field access on h5py groups and files
expr: normalize identifiers to use underscores

Rather than spaces
Merge branch 'tab-complete-on-fields-with-spaces' into h5py-groups
expr: apply valid_identifier only to fields, not all attributes
h5py: hacky compute_down on h5py files/groups
partition: keepdims on slices1d
h5py: robust to chunksizes with singleton dimensions
Merge pull request #818 from mrocklin/h5py-groups

Improve discovery and navigation of h5py files