Added detection of bad index zeroing pulse.

Change-Id: I1607ce0d37b336136a4a278afc8a0f2897d1c597
diff --git a/frc971/constants.h b/frc971/constants.h
index 18e85ae..d66853b 100644
--- a/frc971/constants.h
+++ b/frc971/constants.h
@@ -63,6 +63,9 @@
     double index_difference;
     // The absolute position in scaled units of one of the index pulses.
     double measured_index_position;
+    // Value between 0 and 1 which determines a fraction of the index_diff
+    // you want to use.
+    double allowable_encoder_error;
   };
 
   // Defines a range of motion for a subsystem.