`load_hd5` should have some defaults

[bgrant]

You should be able to load in an hdf5 file without providing a Distribution explicitly, like we allow with fromndarray.

[bgrant]

@markkness' solution:

with h5py.File(pathname, 'r') as f:
    dataset = f[key]
    shape = dataset.shape

Unfortunately, in general we can't assume that the client has access to the file... I suppose engine 0 could get the filename and broadcast it...