added Vertices() to Graph interface

[jonbrandenburg]

This is a proposed solution to #130. Alternatively I could see renaming this to ListVertices() and adding a Vertices() that returns the actual vertices, though that might be more challenging since there is currently no Vertex type so conveying the VertexProperties would not happen as it does with Edges() and their corresponding EdgeProperties.

Building on the earlier idea, it might be beneficial to introduce a Vertex type:

type Vertex[T any] struct {
    Value T
    Properties VertexProperties
}

Then this would enable the ability to create a Vertices() like this:

func Vertices() ([]Vertex[T], error)

[dominikbraun]

Thanks! Yes, a Vertex type would be more convenient for the use case where all full vertex instances should be retrieved, but that's a too large of a change right now. If one really needs this functionality, this should do the job:

for hash, _ := range g.AdjacencyMap() {
    vertex, properties, _ := g.VertexWithProperties(hash)
}

[alexnguyenatwork]

should this read K rather than Vertice[K]?

[dominikbraun]

should this read K rather than Vertice[K]?

@alexnguyenatwork Yes. There is no Vertex type (let alone Vertice).

[AkashKumar7902]

Hey @dominikbraun,
can this be merged ? I want this method more than ever.

[trstruth]

@dominikbraun another friendly ping wondering if it's possible to merge this yet?

I'm able to get the vertices by doing something like

	adjMap, err := g.graph.AdjacencyMap()
	if err != nil {
		return nil, err
	}

	vertices := make(K, 0, len(adjMap))
	i := 0
	for hash := range adjMap {
		vertex, err := g.graph.Vertex(hash)
		if err != nil {
			return nil, err
		}
		vertices[i] = vertex
		i++
	}

but it'd be nice to get this with a function public in the API