better negative sampling + remove_multi_edges + is_bidirected + DotDecoder

[CarloLucibello]

TODO

• add bidirectional sampling option
• add tests for negative_sample

[codecov]

Codecov Report

Merging #69 (e4b7557) into master (e5e919c) will decrease coverage by 0.62%.
The diff coverage is 38.59%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   80.10%   79.47%   -0.63%     
==========================================
  Files          12       12              
  Lines         794      838      +44     
==========================================
+ Hits          636      666      +30     
- Misses        158      172      +14     

Impacted Files	Coverage Δ
src/GNNGraphs/generate.jl	100.00% <ø> (ø)
src/GNNGraphs/gnngraph.jl	67.24% <0.00%> (-4.99%)	⬇️
src/layers/basic.jl	48.78% <ø> (-3.86%)	⬇️
src/GNNGraphs/transform.jl	81.69% <43.58%> (-1.04%)	⬇️
src/GNNGraphs/query.jl	86.84% <100.00%> (+0.60%)	⬆️

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