Support edge weights in GCNConv

[CarloLucibello]

Fix #40

[codecov]

Codecov Report

Merging #76 (ce10af3) into master (5d53d05) will decrease coverage by 0.53%.
The diff coverage is 91.76%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
- Coverage   81.53%   81.00%   -0.54%     
==========================================
  Files          13       13              
  Lines         899      979      +80     
==========================================
+ Hits          733      793      +60     
- Misses        166      186      +20     

Impacted Files	Coverage Δ
src/GNNGraphs/convert.jl	79.04% <86.66%> (-9.98%)	⬇️
src/layers/conv.jl	75.00% <87.50%> (-0.32%)	⬇️
src/GNNGraphs/transform.jl	95.09% <90.00%> (-0.51%)	⬇️
src/GNNGraphs/query.jl	91.33% <95.00%> (+3.43%)	⬆️
src/msgpass.jl	85.29% <100.00%> (+1.96%)	⬆️
src/GNNGraphs/gnngraph.jl	60.93% <0.00%> (-3.00%)	⬇️
src/utils.jl	76.27% <0.00%> (+0.40%)	⬆️
... and 2 more

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