`period` function does not work correctly for abstract directed graphs

[simonschoelly]

Currently the period functions seems to work only for SimpleDiGraph. It should either be restricted to that type or implemented in a way that works with other directed AbstractGraph.

How to reproduce:

julia> using Graphs, SimpleWeightedGraphs

julia> g = cycle_digraph(4)
{4, 4} directed simple Int64 graph

julia> gw = SimpleWeightedDiGraph(g)
{4, 4} directed simple Int64 graph with Float64 weights

julia> period(g)
4

julia> period(gw)
ERROR: MethodError: no method matching difference(::SimpleWeightedDiGraph{Int64, Float64}, ::SimpleDiGraph{Int64})
Closest candidates are:
  difference(::T, ::T) where T<:AbstractGraph at ~/.julia/dev/Graphs/src/operators.jl:196
Stacktrace:
 [1] period(::Type{IsDirected{SimpleWeightedDiGraph{Int64, Float64}}}, g::SimpleWeightedDiGraph{Int64, Float64})
   @ Graphs ~/.julia/dev/Graphs/src/connectivity.jl:503
 [2] period(g::SimpleWeightedDiGraph{Int64, Float64})
   @ Graphs ~/.julia/packages/SimpleTraits/l1ZsK/src/SimpleTraits.jl:331
 [3] top-level scope
   @ REPL[20]:1

[etiennedeg]

I think difference should not be defined for AbstractGraph

[simonschoelly]

Yes, these operator functions such as difference is also something we have to deal with in the future. Maybe it would be possible to implement Period without relaying on difference.

[etiennedeg]

I just made a PR doing this. This will even be more performant, as we don't need to allocate a new graph.

[simonschoelly]

Fixed by #90