Replace HDF5 with h5py.Dataset

This removes the `HDF5(DataDescriptor)` class of yore.  We just manipulate raw `h5py.Dataset` objects instead.

```Python
In [2]: resource('myfile.hdf5::/path/to/mydataset', dshape='5*5*int')
Out[2]: <HDF5 dataset "mydataset": shape (5, 5), type "<i4">
```