Implement the contract_size property

jesse/strategies/Strategy.py

@@ -1205,3 +1205,14 @@ class Strategy(ABC):
             return self.position.exchange.futures_leverage
         else:
             raise ValueError('exchange type not supported!')
+
+    @property
+    def contract_size(self):
+        if self.position.exchange.type != 'inverse futures':
+            raise exceptions.InvalidStrategy(
+                'Only inverse futures have access to the contract_size property. You are accessing it from a {} exchange'.format(
+                    self.position.exchange.type
+                )
+            )
+
+        return self.position.exchange.contract_size

jesse/strategies/TestCanDetectInverseFutures/__init__.py

@@ -10,9 +10,7 @@ class TestCanDetectInverseFutures(Strategy):
             assert self.position.exchange.type == 'inverse futures'
             assert self.symbol == 'BTC-PERP'
             assert self.leverage == 2
-            assert self.position.exchange.contract_size == 1
-            # TODO:
-            # assert self.contract_size == 1
+            assert self.contract_size == 1
 
         return False