make (Abstract)FeaturedGraph subtype LightGraphs.AbstractGraphs

[CarloLucibello]

Since FeaturedGraph already implements some of the core LightGraphs methods, it can be convenient to redefine

abstract type AbstractFeaturedGraph <: AbstractGraph{Int} end

so that we can access the full set of LightGraphs functionalities.
This approach is being experimented int FluxML/GeometricFlux.jl#204

[yuehhua]

Oh! So you want to make the same set of APIs as LightGraphs methods do. For compatibility, it is great. However, I haven't checked the APIs defined for AbstractGraph and if these APIs are suitable for AbstractFeaturedGraph. Due to the reason, I haven't decided to make it as a subtype of AbstractGraph.