Store lattice positions as node attributes

[nwlandry]

Related to #179.

When generating triangular lattices, it should be possible to store the lattice positions as attributes for ease of plotting. This should be an optional argument in the lattice generator functions such as ring_lattice.

xgi.ring_lattice(n, d, k, l, store_pos=True)

[maximelucas]

We could follow the NetworkX terminology and call that argument with_positions