Clean up `multiroute_flow` to correct type instability

[gdalle]

Judging by an old comment

LightGraphsFlows.jl/src/multiroute_flow.jl

Line 97 in 40cdca3

### TODO: CLEAN UP THIS FUNCTION AND DOCUMENTATION. THERE SHOULD BE NO NEED TO

it seems the return type of multiroute_flow depends upon its arguments. Maybe there is a way to do this using multiple dispatch instead?