Payment obligations v2

[rdinicut]

Managed to implement the payment obligation contract with an array of structs. This requires the latest version of web3: 1.0.0-beta.36. and I am using 2 version of web3 in truffle. This is also valid for metamask as it does not support web3 1.0.
We need to check that the AbiEncoder from abigen supports this.

This PR includes multiple issues from the backlog:

• Avoid double-minting of payment obligation NFTs #43
  -Check payment obligation mint parameters against business logic #38: It is missing the document type and collaborator and I suggest we open another issue for that or move what remains in Payment obligation minting should check invoice type and collaborator #35 as we can not implement it now
• Introduce list-based mint method parameters privacy-enabled-erc721#6 : pragma experimental ABIEncoderV2; and web3: 1.0.0-beta.36 required
• Store payment obligation clear text mint params with the token #37

[xmxanuel]

I tried to generate the go-bindings with abigen.
(I used the latest version of abigen from the go-ethereum master branch.)

It seems like it is currently not supported to return an array of structs.

command is used

➜  centrifuge-ethereum-contracts git:(51aba7c) ✗ abigen --abi PaymentObligation.abi --pkg nft --type PaymentObligation --out payment_obligation_ethereum.go
Failed to generate ABI binding: unsupported arg type: tuple

error message

Failed to generate ABI binding: unsupported arg type: tuple

Related issues on go-ethereum

ethereum/go-ethereum#17152
ethereum/go-ethereum#17642