fun <P : AbstractParty, T : Any> sumAmountsDue(balances: Map<Pair<P, P>, Amount<T>>): Map<P, Long>
Calculate the total balance movement for each party in the transaction, based off a summary of balances between each obligor and beneficiary.
balances - payments due, indexed by obligor and beneficiary. Zero balances are stripped from the map before being
P - type of party to operate on.
T - token that balances represent