Improving line following

This makes it so that we try to drive the robot to have the edge of the
hatch panel hit the edge of the velcro on the target.

Also contains a few other tweaks that happened in the interim.

Change-Id: I2d2bffa2b88423b9ac94107daaa0602892ff4da2
diff --git a/frc971/control_loops/drivetrain/drivetrain.q b/frc971/control_loops/drivetrain/drivetrain.q
index 74be414..7f778f7 100644
--- a/frc971/control_loops/drivetrain/drivetrain.q
+++ b/frc971/control_loops/drivetrain/drivetrain.q
@@ -67,6 +67,14 @@
   float x;
   float y;
   float theta;
+  // Current lateral offset from line pointing straight out of the target.
+  float offset;
+  // Current distance from the plane of the target, in meters.
+  float distance_to_target;
+  // Current goal heading.
+  float goal_theta;
+  // Current relative heading.
+  float rel_theta;
 };
 
 queue_group DrivetrainQueue {