Implement sp.Sum, sp.Product

Sum and Product have a indexing variable which is a Atom but not a free symbol. So logic, that defines the undefined symbols in a SympyAssignment should not be atoms(sp.Symbol) but free_symbols. sp.Indexed from the ResolvedFieldAcesses forms an edge case. So we could also use atoms(sp.Symbol).intersection(...free_symbols).

I hope I extracted from my fork all the necessary code to implement this feature.