Fix relative negation of terms for exchangable fermions

[AntonReinhard]

Currently, every diagram is being added together. However, the Feynman rules say that diagrams that can be transformed into one another by exchanging two fermions with one another have to have a relative negation.

In the code, this can be accomplished by inserting minus signs in the CollectPairs and CollectTriples tasks where necessary, i.e., where the Caley distance of the fermion permutations to the identity permutation of the different subdiagrams is uneven.