Allow passing a `Graphs.SimpleGraph` to `siteinds`

[mtfishman]

Allow passing Graphs.SimpleGraph to siteinds(site_type::String, graph).

Also define a default constructor NamedDimGraph(g::SimpleGraph) = NamedDimGraph(g, 1:nv(g)) which could be used internally in siteinds(::String, graph).

Originally posted by @mtfishman in #6 (comment)

[leburgel]

It seems the constructor NamedDimGraph(g::SimpleGraph) already works thanks to NamedGraphs.default_vertices.