Move all vertex definitions to graph.go.
Make source and sinke "visible".
Hide AddSimpleEdge.
Initial support for edge demands.
Introduce edge types.
Fix Dot output.
Don't create excessive edges from Source and to Sink.
Use different style for "demand" edges.
Create correct demand edges.
Correct flow calculation for edges with demand.

Now it has correct value event if graph is not saturated.
Calculate if edge is saturated using values adjusted for demand.
Don't create useless edges from Source to Sink.
Show edge labels where demand is not satisfied in red.
Don't return demand-edges from EdgesFromVertex.
Use flow feasibility to determine if RI exists.