Sets address value to storage yToken.
Usage
const newYTtoken = "KT1..."; // or new BigNumber(0) or "0"
const proxy = await tezos.contract.at(proxyAddress);
const operation = await yupana.methods
.updateYToken(newYTtoken)
.send();
await operation.confirmation();
new_ytoken = "KT1..."
proxy = ContractInterface.from_michelson(code) # or client.contract(contract_address)...
call = proxy.updateYToken(new_ytoken)
opg = call.inject()