Disallow multiple outflows from Outlet at validation (#1570)

Fixes
#1567 (comment)

Author: visr

core/src/validation.jl

@@ -56,7 +56,7 @@ n_neighbor_bounds_flow(::Val{:LevelBoundary}) =
     n_neighbor_bounds(0, typemax(Int), 0, typemax(Int))
 n_neighbor_bounds_flow(::Val{:FlowBoundary}) = n_neighbor_bounds(0, 0, 1, typemax(Int))
 n_neighbor_bounds_flow(::Val{:Pump}) = n_neighbor_bounds(1, 1, 1, typemax(Int))
-n_neighbor_bounds_flow(::Val{:Outlet}) = n_neighbor_bounds(1, 1, 1, typemax(Int))
+n_neighbor_bounds_flow(::Val{:Outlet}) = n_neighbor_bounds(1, 1, 1, 1)
 n_neighbor_bounds_flow(::Val{:Terminal}) = n_neighbor_bounds(1, typemax(Int), 0, 0)
 n_neighbor_bounds_flow(::Val{:PidControl}) = n_neighbor_bounds(0, 0, 0, 0)
 n_neighbor_bounds_flow(::Val{:DiscreteControl}) = n_neighbor_bounds(0, 0, 0, 0)