Combinatorics: added copy(::Graph)

[YueRen]

Many operations for graphs work inplace, so I think it would be nice if there was a copy command for graphs:

julia> G = graph_from_edges([[1,2]])
Undirected graph with 2 nodes and the following edges:
(2, 1)

julia> H = G
Undirected graph with 2 nodes and the following edges:
(2, 1)

julia> collect(edges(H))
1-element Vector{Edge}:
 Edge(2, 1)

julia> rem_edge!(G,1,2)
true

julia> collect(edges(H))
Edge[]