Add pre_order_dfs_vertices

[LinjianMa]

This function is needed when I implement the density matrix approximation algorithm in ITensorNetworks

[mtfishman]

Great, thanks. Could you add a short test?

[LinjianMa]

Great, thanks. Could you add a short test?

Sure, done

[codecov-commenter]

Codecov Report

Merging #30 (f407ac5) into main (35be64d) will increase coverage by 1.29%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   77.39%   78.69%   +1.29%     
==========================================
  Files          19       19              
  Lines         792      798       +6     
==========================================
+ Hits          613      628      +15     
+ Misses        179      170       -9     

Impacted Files	Coverage Δ
src/NamedGraphs.jl	100.00% <ø> (ø)
src/Graphs/abstractgraph.jl	59.77% <100.00%> (+6.79%)	⬆️

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