Add methods for SimpleGraphs and associate tests

[vlandau]

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[codecov-commenter]

Codecov Report

Merging #3 (9b4a288) into main (6ed35e7) will increase coverage by 2.59%.
The diff coverage is 86.53%.

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
+ Coverage   70.99%   73.58%   +2.59%     
==========================================
  Files           5        5              
  Lines         131      159      +28     
==========================================
+ Hits           93      117      +24     
- Misses         38       42       +4     

Impacted Files	Coverage Δ
src/rastergraphs.jl	92.43% <86.53%> (-2.13%)	⬇️
src/structs.jl	100.00% <0.00%> (ø)

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