implement unbatch and add_nodes

[CarloLucibello]

plus some changes to rand_graphs and docstrings improvements

[codecov]

Codecov Report

Merging #65 (0f1e8d4) into master (195bb6c) will decrease coverage by 0.04%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
- Coverage   81.39%   81.35%   -0.05%     
==========================================
  Files          12       12              
  Lines         758      767       +9     
==========================================
+ Hits          617      624       +7     
- Misses        141      143       +2     

Impacted Files	Coverage Δ
src/GNNGraphs/gnngraph.jl	72.22% <ø> (ø)
src/GNNGraphs/query.jl	86.23% <ø> (ø)
src/GNNGraphs/transform.jl	89.21% <71.42%> (-1.41%)	⬇️
src/GNNGraphs/generate.jl	100.00% <100.00%> (ø)

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