Improve support in conv layers for edge features

[CarloLucibello]

Related to #128

[codecov]

Codecov Report

Merging #130 (d2a6188) into master (cf1600e) will increase coverage by 0.37%.
The diff coverage is 84.50%.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   83.75%   84.12%   +0.37%     
==========================================
  Files          15       15              
  Lines        1237     1266      +29     
==========================================
+ Hits         1036     1065      +29     
  Misses        201      201              

Impacted Files	Coverage Δ
src/deprecations.jl	90.00% <ø> (ø)
src/layers/conv.jl	78.33% <84.50%> (+1.71%)	⬆️
src/GNNGraphs/utils.jl	74.74% <0.00%> (+1.01%)	⬆️

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