Add zeroing for single-axis gyro in drivetrain

This ensures that if the pico gets a bad zero we still handle it
properly on the roboRIO.

Change-Id: Ia920ab5698974d4c672a219f01303c404eb4fcb5
Signed-off-by: James Kuszmaul <jabukuszmaul@gmail.com>
diff --git a/frc971/control_loops/drivetrain/drivetrain.h b/frc971/control_loops/drivetrain/drivetrain.h
index 52ae605..a06b965 100644
--- a/frc971/control_loops/drivetrain/drivetrain.h
+++ b/frc971/control_loops/drivetrain/drivetrain.h
@@ -138,6 +138,9 @@
   // Last applied voltage.
   Eigen::Matrix<double, 2, 1> last_voltage_;
   Eigen::Matrix<double, 2, 1> last_last_voltage_;
+
+  std::optional<double> yaw_gyro_zero_;
+  zeroing::Averager<double, 200> yaw_gyro_zeroer_;
 };
 
 class DrivetrainLoop