Split UpdateY from Update in StateFeedbackLoop

Change-Id: Ie821842f23186ff5e49f78ff5476b0120e1f61e2
diff --git a/frc971/control_loops/state_feedback_loop.h b/frc971/control_loops/state_feedback_loop.h
index 6d05444..891aaa6 100644
--- a/frc971/control_loops/state_feedback_loop.h
+++ b/frc971/control_loops/state_feedback_loop.h
@@ -160,6 +160,11 @@
     // that the plant should deal with.
     CheckU(U);
     X_ = Update(X(), U);
+    UpdateY(U);
+  }
+
+  // Computes the new Y given the control input.
+  void UpdateY(const Eigen::Matrix<double, number_of_inputs, 1> &U) {
     Y_ = C() * X() + D() * U;
   }