support for MLDatasets v0.6

[CarloLucibello]

update examples and implement the mldataset2gnngraph convenience method.

Close #162

[codecov]

Codecov Report

Merging #164 (f0d60c6) into master (eb43bc9) will decrease coverage by 0.06%.
The diff coverage is 70.58%.

@@            Coverage Diff             @@
##           master     #164      +/-   ##
==========================================
- Coverage   85.45%   85.38%   -0.07%     
==========================================
  Files          14       15       +1     
  Lines        1299     1300       +1     
==========================================
  Hits         1110     1110              
- Misses        189      190       +1     

Impacted Files	Coverage Δ
src/GNNGraphs/gnngraph.jl	74.66% <0.00%> (-1.34%)	⬇️
src/mldatasets.jl	91.66% <91.66%> (ø)
src/layers/basic.jl	48.78% <100.00%> (ø)
src/GNNGraphs/convert.jl	90.08% <0.00%> (-0.32%)	⬇️
src/GNNGraphs/query.jl	92.98% <0.00%> (-0.05%)	⬇️
src/GNNGraphs/transform.jl	96.71% <0.00%> (-0.04%)	⬇️
src/GNNGraphs/sampling.jl	100.00% <0.00%> (ø)
src/layers/conv.jl	77.31% <0.00%> (+0.07%)	⬆️

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