Fix: It was possible for TD to accept an empty cargo load as an optio…

…n by mistake.

tradecalc.py

@@ -29,7 +29,10 @@
 from tradedb import System, Station
 from collections import namedtuple
 
-TradeLoad = namedtuple('TradeLoad', [ 'items', 'gainCr', 'costCr', 'units' ])
+class TradeLoad(namedtuple('TradeLoad', [ 'items', 'gainCr', 'costCr', 'units' ])):
+    def __bool__(self):
+        return self.units > 0
+
 emptyLoad = TradeLoad([], 0, 0, 0)
 
 TradeHop = namedtuple('TradeHop', [ 'destSys', 'destStn', 'load', 'gainCr', 'jumps', 'ly' ])