Add support for dict-of-dict for add_nodes_from #752

When I run

H = xgi.Hypergraph()
node_dict = {0: {"name": "tom"}}
H.add_nodes_from(node_dict)

and run H.nodes[0], I get {}.

When I run

H = xgi.Hypergraph()
node_dict = {0: {"name": "tom"}}
H.add_nodes_from(node_dict)
H.set_node_attributes(node_dict)

and run H.nodes[0], I get {"name": "tom"}.

I think that add_nodes_from should have the same effect as running the second code snippen.

In the past, we've talked about limiting the number of input formats that we support, but I think that aside from a list/array of nodes to add, this is the second most useful option.