allow one-directional outflow to LevelBoundary (#327)

This came up in a model from @d2hydro
(d2hydro/lhm-ribasim#5 (comment)).

FractionalFlow, FlowBoundary and TabulatedRatingCurve should be able to
flow out into a LevelBoundary. This is slightly odd since in these cases
the level of the level boundary does not matter, since the flow is
always into the LevelBoundary as determined by the other side. So using
a Terminal might make more sense there.

Still, validation shouldn't block this since you may want a single
boundary node that connects to multiple node types, some directed like
this, some with resistance for which you need a level.

core/src/validation.jl

@@ -36,12 +36,15 @@ neighbortypes(::Val{:Basin}) = Set((
     :FlowBoundary,
 ))
 neighbortypes(::Val{:Terminal}) = Set{Symbol}() # only endnode
-neighbortypes(::Val{:FractionalFlow}) = Set((:Basin, :FractionalFlow, :Terminal))
-neighbortypes(::Val{:FlowBoundary}) = Set((:Basin, :FractionalFlow, :Terminal))
+neighbortypes(::Val{:FractionalFlow}) =
+    Set((:Basin, :FractionalFlow, :Terminal, :LevelBoundary))
+neighbortypes(::Val{:FlowBoundary}) =
+    Set((:Basin, :FractionalFlow, :Terminal, :LevelBoundary))
 neighbortypes(::Val{:LevelBoundary}) = Set((:LinearResistance, :ManningResistance))
 neighbortypes(::Val{:LinearResistance}) = Set((:Basin, :LevelBoundary))
 neighbortypes(::Val{:ManningResistance}) = Set((:Basin, :LevelBoundary))
-neighbortypes(::Val{:TabulatedRatingCurve}) = Set((:Basin, :FractionalFlow, :Terminal))
+neighbortypes(::Val{:TabulatedRatingCurve}) =
+    Set((:Basin, :FractionalFlow, :Terminal, :LevelBoundary))
 neighbortypes(::Any) = Set{Symbol}()
 
 # TODO NodeV1 and EdgeV1 are not yet used