MAINT: Always defer to the algorithms data frequency.

For compatibility with existing behavior of having the,
data_frequency of the algorithm override the simulation parameters.

We may want to consider throwing an exception if the two do not match.

zipline/algorithm.py

@@ -144,7 +144,7 @@ class TradingAlgorithm(object):
         if self.sim_params:
             if self.data_frequency is None:
                 self.data_frequency = self.sim_params.data_frequency
-            elif self.sim_params.data_frequency is None:
+            else:
                 self.sim_params.data_frequency = self.data_frequency
 
             self.perf_tracker = PerformanceTracker(self.sim_params)