Implement next_nearest_neighbors

[leburgel]

Implement next_nearest_neighbors(g, v) which returns the next-to-nearest neighbors of a vertex v in g.

It looks like we could make use of Graphs.neighborhood_dists to create a more general function for outputting the vertices that are the k-nearest neighbors of a specified vertex, for example:

function k_nearest_neighbors(g::AbstractGraph, v, d)
  nds = neighborhood_dists(g, v, d)
  # Filter `nds` for vertices with distances equal to `d`
end

Then next_nearest_neighbors(g, v) = k_nearest_neighbors(g, v, 2).

Originally posted by @mtfishman in ITensor/ITensorNetworks.jl#18 (comment)

[mtfishman]

Note this is implemented in ITensorNetworks but just needs to be moved over here.

[mtfishman]

Here are the ITensorNetworks implementations: https://github.com/mtfishman/ITensorNetworks.jl/blob/v0.8.1/src/ModelHamiltonians/ModelHamiltonians.jl#L18-L27