Add check diagonal

Author: lkdvos

src/spaces/homspace.jl

@@ -354,6 +354,8 @@ end
 #----------------
 # TODO: is this something we want to cache?
 function diagonalblockstructure(W::HomSpace)
+    ((numin(W) == numout(W) == 1) && domain(W) == codomain(W)) ||
+        throw(ArgumentError("Input space is not diagonal"))
     structure = SectorDict{sectortype(W),UnitRange{Int}}() # range
     offset = 0
     dom = domain(W)[1]