add compat for Graphs.jl objects

[hexaeder]

closes #42

Since there is just a compat layer using Requires it should work now for both: Graphs.jl and LightGraphs.jl users

[codecov]

Codecov Report

Merging #43 (88b727d) into master (f1b3f76) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #43   +/-   ##
=======================================
  Coverage   98.01%   98.02%           
=======================================
  Files           8        8           
  Lines         454      455    +1     
=======================================
+ Hits          445      446    +1     
  Misses          9        9           

Impacted Files	Coverage Δ
src/NetworkLayout.jl	94.28% <100.00%> (+0.16%)	⬆️

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