Make arm zeroing safer.

Now it forces the elevator to go to a predefined "safe" height
before it zeroes the arm to avoid crashing the fridge.

Change-Id: I8b5197a6a1e3d75372756015026c5ea8d1ddddef
diff --git a/frc971/control_loops/fridge/fridge.h b/frc971/control_loops/fridge/fridge.h
index 54dc032..fc10594 100644
--- a/frc971/control_loops/fridge/fridge.h
+++ b/frc971/control_loops/fridge/fridge.h
@@ -18,6 +18,7 @@
 class FridgeTest_ElevatorGoalPositiveWindupTest_Test;
 class FridgeTest_ArmGoalNegativeWindupTest_Test;
 class FridgeTest_ElevatorGoalNegativeWindupTest_Test;
+class FridgeTest_SafeArmZeroing_Test;
 }
 
 class CappedStateFeedbackLoop : public StateFeedbackLoop<4, 2, 2> {
@@ -74,6 +75,7 @@
   friend class testing::FridgeTest_ArmGoalPositiveWindupTest_Test;
   friend class testing::FridgeTest_ElevatorGoalNegativeWindupTest_Test;
   friend class testing::FridgeTest_ArmGoalNegativeWindupTest_Test;
+  friend class testing::FridgeTest_SafeArmZeroing_Test;
 
   // Sets state_ to the correct state given the current state of the zeroing
   // estimators.