better printing for GNNGraphs and check that features are arrays

[CarloLucibello]

No description provided.

[codecov]

Codecov Report

Merging #145 (9ba9f5f) into master (a82a320) will increase coverage by 0.12%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
+ Coverage   85.48%   85.61%   +0.12%     
==========================================
  Files          15       15              
  Lines        1247     1251       +4     
==========================================
+ Hits         1066     1071       +5     
+ Misses        181      180       -1     

Impacted Files	Coverage Δ
src/GNNGraphs/gnngraph.jl	80.00% <0.00%> (ø)
src/GNNGraphs/utils.jl	74.48% <100.00%> (+1.29%)	⬆️
src/GNNGraphs/query.jl	92.98% <0.00%> (+0.04%)	⬆️
src/GNNGraphs/convert.jl	90.08% <0.00%> (+0.16%)	⬆️

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