Throw an error if `named_hexagonal_lattice_graph(n, m; periodic = true)` is called with `n` or `m` = 2

[JoeyT1994]

We should throw an error if named_hexagonal_lattice_graph(n, m; periodic = true) is called with n =2 or m = 2 because this will output a graph that essentially is a ladder (with shortest loop size 4) and resembles nothing like a hexagonal graph so may be misleading to the user.

This is likely an issue implicitly tied to the hexagonal lattice itself (hexagonal_lattice_graph for Networkx in Python also has the same output).