Define a generic `is_directed(::Type{<:AbstractDataGraph})`

[mtfishman]

Define a generic is_directed(G::Type{<:AbstractDataGraph}) = is_directed(underlying_graph_type(G)). This would then rely on concrete subtypes implementing underlying_graph_type.

Inspired by ITensor/ITensorNetworks.jl#9, since it seems like in general we should be able to deduce whether a DataGraph is directed or not from other information.

[mtfishman]

Closed by #4.