Default structs to define

[gdalle]

In addition to the interface, this package should probably contain basic implementations for the most common types of graphs.

Here's a possible list:

Name	Vertices	Metadata
(Simple)(Di)Graph	1:n	none
(Simple)Weighted(Di)Graph	1:n	edge weights
(Simple)Meta(Di)Graph	1:n	vertex and edge
(Simple)VertexSafe(Di)Graph	non-contiguous integers	none

For each one, we will provide both the simple version (no multi-edges, possible self-loops) and the generic version.

[filchristou]

I would really suggest to try our best to make MetaGraphs.jl obselete. So this would mean that the implementation of a MetaGraph should be type-stable for performance but also fallback to a generic struct using Any types for flexibility.

[gdalle]

I'm not sure yet how it will look like, but I don't think a fallback is a great idea. I think if users want metadata of type Any, they have to say it explicitly. Strictly typed metadata is also a safeguard against bad code