Rework accumulation splitting

Before this commit splitting into accumulation terms took indices into account.

Example: The expression

\sum_i a_i (\nabla v)_i + \sum_j b_j (\nabla v)_j

got split into two accumulation terms:

a_k for (\nabla v)_k b_l for (\nabla v)_l

Now we only have one:

a_k + b_k for (\nabla v)_k