voluntary Labels support

It would be nice if there was a version of MetaGraphs that didn't need the explicit label definition from the user.

I guess something similar to AttributeGraphs.jl. OAttributeGraph models the vertex properties as a Vector and switches the elements for every vertex deletion, etc.

Here in MetaGraphsNext.jl the data structure is hardcored to be Dict{Label,Tuple{Code,VertexData}}. Ofc the user could make a Label being an Int. But at the moment s/he will also need to maintain it, which is a bother.

@gdalle suggested a MetaGraph subtype with automatic labeling. I am not sure why would you need a subtype ??

I was thinking something like interfacing a keyword in the constructor default_labeling=false that when it's true, we take over and make it so that label::Int=code::Int at any state.

Start by reading the graph constructor and the current Dict{Label,Tuple{Code,VertexData}} storage described in the issue. Compare the proposed default_labeling option with the referenced AttributeGraphs.jl approach and the suggested MetaGraph subtype. Done means automatic integer labels remain consistent through vertex deletion without requiring user-maintained labels.