Clean up `multiroute_flow` to correct type instability

[gdalle]

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Judging by an old comment https://github.com/JuliaGraphs/LightGraphsFlows.jl/blob/40cdca3a3cd6b7c21dd5077306b8d4201ffb172d/src/multiroute_flow.jl#L97 it seems the return type of multiroute_flow depends upon its arguments. Maybe there is a way to do this using multiple dispatch instead?