Put a plant inside the loop and moved A,B,etc there.

Change-Id: I9cb3a1a16bd0ccda0c9287514577b3c3861bc42f
diff --git a/frc971/control_loops/drivetrain/polydrivetrain.h b/frc971/control_loops/drivetrain/polydrivetrain.h
index 20f1a49..570b87f 100644
--- a/frc971/control_loops/drivetrain/polydrivetrain.h
+++ b/frc971/control_loops/drivetrain/polydrivetrain.h
@@ -17,7 +17,7 @@
   PolyDrivetrain(const DrivetrainConfig &dt_config,
                  StateFeedbackLoop<7, 2, 3> *kf);
 
-  int controller_index() const { return loop_->controller_index(); }
+  int controller_index() const { return loop_->index(); }
 
   // Computes the speed of the motor given the hall effect position and the
   // speed of the robot.