Create order

      The amount user want to sell/buy (positive for buy, negative for sell). An order could be partially executed,
      e.g. an order sell 5 btccan be matched with a buy 3 btc order, left 2 btc to be sold;
      in this case the order's volume would be '5.0', its remaining_volume would be '2.0',
      its executed volume is '3.0'.
      

Price for each unit. e.g. If you want to sell/buy 1 btc at 3000 uah, the price is '3000.0'