Common interface for graphs with metadata

Hey!

There has been some talk recently about adding better support for graph metadata, so here's an issue to centralize our discussions. First of all, here are the packages that I'm aware of that deal with vertex- & edge-level attributes:

• https://github.com/JuliaGraphs/SimpleWeightedGraphs.jl
• https://github.com/JuliaGraphs/MetaGraphs.jl
• https://github.com/simonschoelly/SimpleValueGraphs.jl @simonschoelly
• https://github.com/JuliaGraphs/MetaGraphsNext.jl @bramtayl

One idea would be to design a common interface extending AbstractGraph to work with metadata. I'm aware that the four packages above are very different, and that the case of edge weights probably deserves special treatment. On the other hand, agreeing on a common set of names for functions would bring clear benefits:

• User-friendliness would increase significantly, as well as ease of documentation
• Switching between meta-graph implementations would be instantaneous. This is sometimes necessary to improve performance
• Compatibility with downstream packages such as https://github.com/JuliaGraphs/GraphDataFrameBridge.jl would be a non-issue
• The same goes with upstream packages such as https://github.com/JuliaGraphs/GraphIO.jl or https://github.com/JuliaGraphs/SNAPDatasets.jl

As food for thought, here are a few past discussions on this topic:

• https://discourse.julialang.org/t/creating-a-weighted-graph/41241/17
• https://discourse.julialang.org/t/retworkx-new-high-performance-python-graph-library/71109/5

What is your take on this?

The issue names no repository files or tests. Start by reviewing the AbstractGraph extension idea and comparing the metadata approaches in SimpleWeightedGraphs.jl, MetaGraphs.jl, SimpleValueGraphs.jl, and MetaGraphsNext.jl. Done would require agreement on a common metadata interface and its function names, including how edge weights are treated.