implement hash for GNNGraph

[CarloLucibello]

Make sure that g and deepcopy(g) are hashed the same. This is relevant when graphs are used as dictionary keys (e.g. when used as states in the implementation of the MCTS in AlphaZero.jl).

cc @bhalonen

[codecov]

Codecov Report

Merging #121 (f71dceb) into master (2493acf) will increase coverage by 0.52%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   82.97%   83.50%   +0.52%     
==========================================
  Files          15       15              
  Lines        1204     1194      -10     
==========================================
- Hits          999      997       -2     
+ Misses        205      197       -8     

Impacted Files	Coverage Δ
src/GNNGraphs/gnngraph.jl	77.61% <100.00%> (+11.45%)	⬆️
src/GNNGraphs/utils.jl	69.76% <100.00%> (-0.35%)	⬇️
src/utils.jl	75.86% <0.00%> (-0.41%)	⬇️
src/GNNGraphs/convert.jl	89.74% <0.00%> (-0.34%)	⬇️
src/GNNGraphs/query.jl	92.94% <0.00%> (-0.05%)	⬇️
src/GNNGraphs/transform.jl	96.44% <0.00%> (-0.04%)	⬇️
src/GNNGraphs/sampling.jl	100.00% <0.00%> (ø)
src/layers/conv.jl	76.79% <0.00%> (+0.17%)	⬆️

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