Cleanup hypergraph methods

[leotrs]

Continued progress on #94

1. H.isolates is now H.nodes.isolates and returns a NodeView.
2. H.singleton_edges is now H.edges.singletons and returns an EdgeView.

There are two related convenience methods, H.remove_isolates and H.remove_singleton_edges that I have left for now, though I think they should both go in the future in favor of a general cleanup method (see #36).

[nwlandry]

Two thoughts:

1. We already have remove_nodes_from and remove_edges_from so it wouldn't be too terrible to get rid of remove_isolates and remove_singleton_edges, since we could just do H.remove_nodes_from(H.nodes.isolates) and H.remove_edges_from(H.edges.singletons)
2. I would update the docs to match the changed location of these methods.

[nwlandry]

See comment.

[leotrs]

Done!