Add error detection to the absolute encoder zeroing

Change-Id: I36ca16ce174c0bd459f2d649fdae607b5813f2c5
diff --git a/frc971/constants.h b/frc971/constants.h
index 57637dd..7b4cb47 100644
--- a/frc971/constants.h
+++ b/frc971/constants.h
@@ -38,12 +38,15 @@
   // Measured absolute position of the encoder when at zero.
   double measured_absolute_position;
 
-  // Treshold for deciding if we are moving
-  // TODO(austin): Figure out what this is actually measuring.
+  // Treshold for deciding if we are moving. moving_buffer_size samples need to
+  // be within this distance of each other before we use the middle one to zero.
   double zeroing_threshold;
-
   // Buffer size for deciding if we are moving.
   size_t moving_buffer_size;
+
+  // Value between 0 and 1 indicating what fraction of one_revolution_distance
+  // it is acceptable for the offset to move.
+  double allowable_encoder_error;
 };
 
 // Defines a range of motion for a subsystem.