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_states.h b/frc971/control_loops/drivetrain/drivetrain_states.h
new file mode 100644
index 0000000..9ea2518
--- /dev/null
+++ b/frc971/control_loops/drivetrain/drivetrain_states.h
@@ -0,0 +1,24 @@
+#ifndef FRC971_CONTROL_LOOPS_DRIVETRAIN_DRIVETRAIN_STATES_H_
+#define FRC971_CONTROL_LOOPS_DRIVETRAIN_DRIVETRAIN_STATES_H_
+
+namespace frc971 {
+namespace control_loops {
+namespace drivetrain {
+
+enum KalmanState {
+  kLeftPosition = 0,
+  kLeftVelocity = 1,
+  kRightPosition = 2,
+  kRightVelocity = 3,
+  kLeftError = 4,
+  kRightError = 5,
+  kAngularError = 6
+};
+
+enum OutputState { kLeftVoltage = 0, kRightVoltage = 1 };
+
+}  // namespace drivetrain
+}  // namespace control_loops
+}  // namespace frc971
+
+#endif  // FRC971_CONTROL_LOOPS_DRIVETRAIN_DRIVETRAIN_STATES_H_