Add the ability to view/load single datasets in a data collection

[nwlandry]

From #540: "xgi.load_xgi_data("hyperbard") loads a dict of datasets, so xgi.load_xgi_data("hyperbard")["coriolanus"] should load a single dataset if I understand correctly...I'm wondering if don't want to be able to access them directly from something like xgi.load_xgi_data("hyperbard-coriolanus") to be able to iterate over all datasets in XGI-data. They could also appear in the list we get with xgi.load_xgi_data() this way?"