add radius_graph api

[YichengDWu]

Close #177

[codecov]

Codecov Report

Merging #180 (0ab69ea) into master (78792f0) will increase coverage by 0.26%.
The diff coverage is 100.00%.

❗ Current head 0ab69ea differs from pull request most recent head 20739e4. Consider uploading reports for the commit 20739e4 to get more accurate results

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   86.15%   86.42%   +0.26%     
==========================================
  Files          15       15              
  Lines        1336     1333       -3     
==========================================
+ Hits         1151     1152       +1     
+ Misses        185      181       -4     

Impacted Files	Coverage Δ
src/GNNGraphs/generate.jl	100.00% <100.00%> (ø)
src/GNNGraphs/convert.jl	90.08% <0.00%> (-0.32%)	⬇️
src/GNNGraphs/transform.jl	96.91% <0.00%> (-0.06%)	⬇️
src/GNNGraphs/query.jl	93.64% <0.00%> (-0.04%)	⬇️
src/GNNGraphs/sampling.jl	100.00% <0.00%> (ø)
src/layers/conv.jl	77.31% <0.00%> (+0.07%)	⬆️
src/GNNGraphs/gnngraph.jl	75.00% <0.00%> (+2.50%)	⬆️

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[CarloLucibello]

Thank you!