implement DataStore

[CarloLucibello]

Provide more flexibility to GNNGraph by storing features into a custom class, the DataStore, with an interface inheriting from both dictionaries and named tuples. We can now mutate the graph storing new features arrays with

g.ndata.z = rand(3, 10)

TODO

• [x] fix broken tests
• [x] add more test to cover the new features
• [x] extend to heterographs
• [x] docs

[CarloLucibello]

tests are passing locally but for some reason they are not passing on the CI

[codecov[bot]]

Codecov Report

Merging #232 (0c7e043) into master (1381a78) will decrease coverage by 7.58%. The diff coverage is 58.23%.

:exclamation: Current head 0c7e043 differs from pull request most recent head 41315b1. Consider uploading reports for the commit 41315b1 to get more accurate results

@@            Coverage Diff             @@
##           master     #232      +/-   ##
==========================================
- Coverage   88.29%   80.70%   -7.59%     
==========================================
  Files          16       17       +1     
  Lines        1597     1731     +134     
==========================================
- Hits         1410     1397      -13     
- Misses        187      334     +147     

Impacted Files	Coverage Δ
src/GNNGraphs/gnnheterograph.jl	23.94% <0.00%> (-57.88%)	:arrow_down:
src/GNNGraphs/gnngraph.jl	55.11% <25.00%> (-44.07%)	:arrow_down:
src/GNNGraphs/gatherscatter.jl	75.00% <50.00%> (-5.00%)	:arrow_down:
src/GNNGraphs/datastore.jl	72.72% <72.72%> (ø)
src/GNNGraphs/utils.jl	75.40% <82.14%> (-16.34%)	:arrow_down:
src/GNNGraphs/query.jl	94.02% <100.00%> (ø)
src/GNNGraphs/sampling.jl	100.00% <100.00%> (ø)
src/GNNGraphs/transform.jl	96.55% <100.00%> (+0.43%)	:arrow_up:
... and 2 more

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