Use enums for drivetrain KF states

This makes things a lot easier to follow.

Change-Id: Ib3d2b60fd8f77851cf842340447d92905b69cb75
diff --git a/frc971/control_loops/drivetrain/drivetrain.h b/frc971/control_loops/drivetrain/drivetrain.h
index d350321..f282cc0 100644
--- a/frc971/control_loops/drivetrain/drivetrain.h
+++ b/frc971/control_loops/drivetrain/drivetrain.h
@@ -11,6 +11,7 @@
 #include "frc971/control_loops/drivetrain/drivetrain_goal_generated.h"
 #include "frc971/control_loops/drivetrain/drivetrain_output_generated.h"
 #include "frc971/control_loops/drivetrain/drivetrain_position_generated.h"
+#include "frc971/control_loops/drivetrain/drivetrain_states.h"
 #include "frc971/control_loops/drivetrain/drivetrain_status_generated.h"
 #include "frc971/control_loops/drivetrain/gear.h"
 #include "frc971/control_loops/drivetrain/improved_down_estimator.h"