Update the proofs to allow for multiple in and out payments

[petermunnings]

For the MAS project we need to generate proofs for multiple inputs and multiple outputs as per this document:

https://docs.google.com/document/d/1Awn4jTfkVVF_fFsxnbK2wLxqEc2hhDvoXAEZvnv-zSM/edit#heading=h.1z36lnpleyya

[petermunnings]

The first step has been completed. You can now have both an incoming and outgoing payment in the same proof.