STY: Uses if/else for setting sim_params in algorithm.

Instead of checking 'if' and then 'if not', uses an if/else.

zipline/algorithm.py

@@ -197,8 +197,7 @@ class TradingAlgorithm(object):
 
         if sim_params:
             self.sim_params = sim_params
-
-        if not self.sim_params:
+        else:
             self.sim_params = create_simulation_parameters(
                 start=start,
                 end=end,