fix stack broadcast dims performance where possible (#767)

* fix stack broadcast dims performance where possible

* method not needed

* manually throw error

src/utils.jl

@@ -130,8 +130,8 @@ end
 function broadcast_dims(f, As::Union{AbstractDimStack,AbstractBasicDimArray}...)
     st = _firststack(As...)
     nts = _as_extended_nts(NamedTuple(st), As...)
-    layers = map(nts...) do as...
-        broadcast_dims(f, as...)
+    layers = map(keys(st)) do name
+        broadcast_dims(f, map(nt -> nt[name], nts)...)
     end
     rebuild_from_arrays(st, layers)
 end
@@ -214,9 +214,14 @@ uniquekeys(nt::NamedTuple) = keys(nt)
 
 _as_extended_nts(nt::NamedTuple{K}, A::AbstractDimArray, As...) where K = 
     (NamedTuple{K}(ntuple(x -> A, length(K))), _as_extended_nts(nt, As...)...)
-function _as_extended_nts(nt::NamedTuple{K}, st::AbstractDimStack, As...) where K
+function _as_extended_nts(nt::NamedTuple{K1}, st::AbstractDimStack{K2}, As...) where {K1,K2}
+    K1 == K2 || throw(ArgumentError("Keys of stack $K2 do not match the keys of the first stack $K1"))
     extended_layers = map(layers(st)) do l
-        DimExtensionArray(l, dims(st))
+        if all(hasdim(l, dims(st)))
+            l
+        else
+            DimExtensionArray(l, dims(st))
+        end
     end
     return (extended_layers, _as_extended_nts(nt, As...)...)
 end