add support for batched graphs

[CarloLucibello]

Fix #12

[codecov]

Codecov Report

Merging #17 (3043cce) into master (b4a8675) will increase coverage by 0.89%.
The diff coverage is 90.32%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   73.72%   74.61%   +0.89%     
==========================================
  Files           7        7              
  Lines         430      453      +23     
==========================================
+ Hits          317      338      +21     
- Misses        113      115       +2     

Impacted Files	Coverage Δ
src/layers/pool.jl	94.73% <83.33%> (-5.27%)	⬇️
src/gnngraph.jl	84.39% <91.30%> (+1.75%)	⬆️
src/utils.jl	100.00% <100.00%> (ø)

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