Add J and Mass to drivetrain_config

They are needed for trajectory planning.

Change-Id: I8daf4303fee347af77567b20078f597354ba2472
diff --git a/frc971/control_loops/drivetrain/drivetrain_config.h b/frc971/control_loops/drivetrain/drivetrain_config.h
index c4652be..88547d5 100644
--- a/frc971/control_loops/drivetrain/drivetrain_config.h
+++ b/frc971/control_loops/drivetrain/drivetrain_config.h
@@ -72,6 +72,10 @@
   Scalar high_gear_ratio;
   Scalar low_gear_ratio;
 
+  // Moment of inertia and mass.
+  Scalar J;
+  Scalar mass;
+
   // Hall effect constants. Unused if not applicable to shifter type.
   constants::ShifterHallEffect left_drive;
   constants::ShifterHallEffect right_drive;