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/constants.h b/frc971/constants.h
index 9e2ccf6..e12a27f 100644
--- a/frc971/constants.h
+++ b/frc971/constants.h
@@ -102,6 +102,9 @@
     double right_elevator_potentiometer_offset;
     double left_arm_potentiometer_offset;
     double right_arm_potentiometer_offset;
+
+    // How high the elevator has to be before we start zeroing the arm.
+    double arm_zeroing_height;
   };
   Fridge fridge;