Function for extracting node positions from attributes

[nwlandry]

Related to #623 and #179.

It would be great to have a function to do the following:

H = xgi.ring_lattice(n, d, k, l, store_pos=True)
pos = xgi.pos_from_attributes(H, attr="pos")
xgi.draw(H, pos=pos)

[kaiser-dan]

Since the usual format for the pos argument passed to draw is a dictionary mapping a node id to a 2d-coordinate, could this be a more general function to extract an attribute to a dictionary? Something like

H = xgi.ring_lattice(n, d, k, l, store_pos=True)
attr_dict = xgi.dict_from_attributes(H, attr="pos")
xgi.draw(H, pos=attr_dict)

It seems like the main advantage to a method specifying the retrieved mapping is a coordinate mapping is in implicit validation that the returned values are tuples of numbers. When this is not true, however, the draw function would throw an error regardless, and it seems like the intended use for this issue and the draw function would go hand-in-hand.

Either way, I can start on this today.

[nwlandry]

Yeah, I could see that being useful? One way to handle this is with the stats interface: H.nodes.attrs("pos").asdict(), but we've had lingering issues with the visibility of the stats module.