code: def transfer_to_main(self, amount, currency, subaccount=None):
          
          data = {'amount': amount,
                  'currency': currency,}
          if subaccount is not None:
              data['subAccount'] = subaccount
          return self._post("transfer-to-main/", data=data, return_json=True,
                            version=2)
docs: Returns dictionary with status.
          subaccount has to be the numerical id of the subaccount, not the name