Restrictoplus to ElementarySpace (#265)

* restrict oplus to ElementarySpace

* Add test

Author: lkdvos

src/spaces/vectorspaces.jl

@@ -146,8 +146,8 @@ spaces `V₁`, `V₂`, ... Note that all the individual spaces should have the s
 [`isdual`](@ref), as otherwise the direct sum is not defined.
 """
 function ⊕ end
-⊕(V₁::VectorSpace, V₂::VectorSpace) = ⊕(promote(V₁, V₂)...)
-⊕(V::Vararg{VectorSpace}) = foldl(⊕, V)
+⊕(V₁::ElementarySpace, V₂::ElementarySpace) = ⊕(promote(V₁, V₂)...)
+⊕(V::Vararg{ElementarySpace}) = foldl(⊕, V)
 const oplus = ⊕
 
 """

test/spaces.jl

@@ -323,6 +323,7 @@ println("------------------------------------")
         @test Base.eltype(P) == Base.eltype(typeof(P)) == typeof(V1)
         @test eltype(collect(P)) == typeof(V1)
         @test collect(P) == [V1, V2, V3, V4]
+        @test_throws MethodError P ⊕ P
     end
 
     @timedtestset "ProductSpace{SU₂Space}" begin