Simple way to list available datasets on XGI-data

[lordgrilo]

I'm not sure if this goes here or in the XGI-data repo directly, but it would be useful to have a simple function list the possible datasets that one can download, without having to go and check the index.json file in xgi-data.

[nwlandry]

This is a good idea. I'm not sure how to make this happen but I'll think more about it.