Allow random access in HDF5TableReader

[maxnoe]

Please describe the use case that requires this feature.

The HDF5TableReader essentially uses random access in a generator:

for row_index in range(n_rows):
     row = table[row_index]
     ...
     yield containers

But it doesn't expose this underlying random access nature.

Describe the solution you'd like

Change API of HDF5TableReader to allow __getitem__(self, index) to allow random access.

[kosack]

Just out of curiosity, where would this be useful?