refactor GNNGraph into submodule + implement add_edges

[CarloLucibello]

No description provided.

[codecov]

Codecov Report

Merging #64 (df874da) into master (375b787) will increase coverage by 0.43%.
The diff coverage is 86.14%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
+ Coverage   80.96%   81.39%   +0.43%     
==========================================
  Files           8       12       +4     
  Lines         746      758      +12     
==========================================
+ Hits          604      617      +13     
+ Misses        142      141       -1     

Impacted Files	Coverage Δ
src/GNNGraphs/convert.jl	88.88% <ø> (ø)
src/utils.jl	75.86% <ø> (-7.31%)	⬇️
src/GNNGraphs/gnngraph.jl	72.22% <72.22%> (ø)
src/GNNGraphs/query.jl	86.23% <86.23%> (ø)
src/GNNGraphs/transform.jl	90.62% <90.62%> (ø)
src/GNNGraphs/utils.jl	94.28% <94.28%> (ø)
src/GNNGraphs/generate.jl	100.00% <100.00%> (ø)

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