Document LightweightTableCollection interface and usage

[jeromekelleher]

See #862 for background.

Need to add documentation for how to use the LightweightTableCollection code in downstream projects. Ideally we want to add some minimal examples using the Python-C interface and Cython to our build examples.

[jeromekelleher]

Also the code needs some cleaning up, I dropped a few TODOs around the place.