Sums with different sum indices don't get simplified

[andia89]

@cnumbers N
h = NLevelSpace(:atoms, 2)

i = Index(h,:i,N,h)
j = Index(h,:j,N,h)

sigma(i,j,k) = IndexedOperator(Transition(h,:σ,i,j),k)
sigmam(k) = sigma(1, 2, k)

SymbolicUtils.simplify(∑(sigmam(i), i)+∑(sigmam(j), j))

outputs

(Σ(i=1:N)σ12i+Σ(j=1:N)σ12j)

Even though it should

2(Σ(i=1:N)σ12i)

[ChristophHotter]

We did not implement this simplification rule. However, this is not needed to create the correct equations. When the equations are scaled or evaluated the terms will be simplified.