Publish the x, y, theta state of the drivetrain

This gives us a start of an on-field position to follow.

Change-Id: Ibdd3f06e1856efd451ceb504801ed32605b5e560
diff --git a/frc971/control_loops/drivetrain/drivetrain.q b/frc971/control_loops/drivetrain/drivetrain.q
index 7fb859d..2e78e51 100644
--- a/frc971/control_loops/drivetrain/drivetrain.q
+++ b/frc971/control_loops/drivetrain/drivetrain.q
@@ -145,8 +145,11 @@
     double estimated_angular_velocity_error;
     // The KF estimated heading.
     double estimated_heading;
-    // The KF wheel estimated heading.
-    //double estimated_wheel_heading;
+
+    // xytheta of the robot.
+    double x;
+    double y;
+    double theta;
 
     // True if the output voltage was capped last cycle.
     bool output_was_capped;