Correct type of prefer_forward penalty member variable

The setter and the configuration treat this value as a double, this
makes sense when comparing it to the other costs. However, it was stored
as a bool. This seems like a typo.

base_local_planner/include/base_local_planner/prefer_forward_cost_function.h

@@ -57,7 +57,7 @@ class PreferForwardCostFunction: public base_local_planner::TrajectoryCostFuncti
   }
 
 private:
-  bool penalty_;
+  double penalty_;
 };
 
 } /* namespace base_local_planner */