edges in collect(edges(g)) should be ordered by index

[CarloLucibello]

for the Network type, and more generally for graphs with indexed edges.

Right now edges are iterated in lexycographic order for (src,dst):

julia> g= Network(10,20)
Network(10, 20) with [] graph, [] vertex, [] edge properties

julia> collect(edges(g))
20-element Array{Erdos.IndexedEdge,1}:
 (1=>4,4)  
 (1=>7,11) 
 (1=>5,16) 
 (1=>9,17) 
 (2=>3,7)  
 (2=>9,12) 
 (3=>10,9) 
 (3=>7,10) 
 (3=>8,18) 
 (4=>10,2) 
 (4=>5,5)  
 (4=>6,6)  
 (4=>7,15) 
 (5=>9,1)  
 (5=>6,19) 
 (6=>8,14) 
 (6=>9,20) 
 (7=>9,3)  
 (8=>9,8)  
 (8=>10,13)

[CarloLucibello]

It is impossible to have efficient iteration by index order with the current implementation of Network. Probably this is issue is a won't fix