Iterating through data

I am using torch-hdf5 to exchange big chunks of data between Python and Lua. I want to iterate through the data without loading everything first. I am able to do it with the code below but I am accessing directly (undocumented) member variables. Is there a better way?

``` lua
h5 = hdf5.open('file.h5', 'r')
for k,c in pairs(h5._rootGroup._children) do
local d = h5:read(k):all()
end
```

Start with the public hdf5.open and h5:read APIs shown in the issue, then inspect how datasets are exposed without relying on _rootGroup._children. Determine the supported iteration behavior and document or add an API that avoids loading the full dataset; done means the Lua example can iterate through the data using public interfaces.

Written by the indexing model from the issue text.