add w_mul_xj and set_edge_weight

[CarloLucibello]

Also clean up the GCNConv

Close #110

[codecov]

Codecov Report

Merging #114 (5d10ed9) into master (df5b508) will decrease coverage by 0.69%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
- Coverage   83.30%   82.61%   -0.70%     
==========================================
  Files          14       14              
  Lines        1180     1179       -1     
==========================================
- Hits          983      974       -9     
- Misses        197      205       +8     

Impacted Files	Coverage Δ
src/msgpass.jl	65.30% <28.57%> (-11.62%)	⬇️
src/GNNGraphs/convert.jl	90.08% <50.00%> (ø)
src/GNNGraphs/transform.jl	96.00% <100.00%> (+0.09%)	⬆️
src/layers/conv.jl	76.62% <100.00%> (-1.09%)	⬇️

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