New interface for GraphsExtensions.similar_graph; misc bug fixes.

[jack-dunham]

This PR overhauls the way the GraphsExtensions.similar_graph function behaves, and includes some minor bug fixes and refactors.

New similar_graph interface

Essentially, the base case is now the three argument method, where one provides both vertices and edges. This defaults to constructing a concrete NamedGraph or NamedDiGraph depending on the IsDirected trait of the input graph.

similar_graph(::AbstractGraph::!(IsDirected), vertices, edges) # -> NamedGraph
similar_graph(::AbstractGraph::IsDirected, vertices, edges) # -> NamedDiGraph

If edges are not provided, then the function attempts to construct a similar graph to the input graph, but with the provided vertices and no edges. If no vertices or edges are provided, then the output graph will be a similar empty graph. For a given subtype of AbstractNamedGraph, one should overload the three argument method only (if a different return type is desired).

Type domain

If instead a graph type is provided as the first argument to similar_graph, then the constructor is called by default:

similar_graph(T::Type{<:AbstractGraph})  = T()
similar_graph(T::Type{<:AbstractGraph}, vertices)  = T(vertices)
similar_graph(T::Type{<:AbstractGraph}, vertices, edges)  = T(vertices, edges)

Note, unlike the case where a graph value is provided, the one and two argument methods are not defined in terms of the three argument method. This is to accommodate cases where a given AbstractGraph may have fixed edges, say (a notable example would be a tree graph). In that case, it is assumes there exists a constructor T(vertices). As the method

similar_graph(T::Type{<:AbstractGraph}, vertices)  = T(vertices)

may or may not return an edgeless graph, depending on the definition of the constructor, there exists the function

edgeless_graph(T::Type{<:AbstractGraph}, vertices)

that attempts to explicitly a construct an edgeless graph, thus throwing an error when this is not possible. Likewise, there exists a function

empty_graph(T::Type{<:AbstractGraph})

for explicitly constructing a graph of type T with no vertices or edges.

If your graph type MyGraph has freedom over vertices and edges, all three type-based method can be overloaded simultaneously using the followings signature:

similar_graph(T::Type{<:MyGraph}, vertices = [], edges = [])  = ...

[mtfishman]

Likewise, there exists a function

empty_graph(T::Type{<:AbstractGraph}, vertices)

for explicitly constructing a graph of type T with no vertices or edges.

Is that supposed to be:

empty_graph(T::Type{<:AbstractGraph})

?