Add JAX version of the physics, and tests to confirm it matches

This sets us up to very efficiently compute dynamics and jit.

Change-Id: I57aea5c1f480759c8e5e658ff6f4de0d82ef273d
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/frc971/control_loops/swerve/generate_physics.cc b/frc971/control_loops/swerve/generate_physics.cc
index 3c4eaf6..e796ec6 100644
--- a/frc971/control_loops/swerve/generate_physics.cc
+++ b/frc971/control_loops/swerve/generate_physics.cc
@@ -142,7 +142,7 @@
       rb1_ = symbol("rb1");
       rb2_ = symbol("rb2");
 
-      Gd2_ = symbol("Gd3");
+      Gd3_ = symbol("Gd3");
       Gd_ = symbol("Gd");
 
       Js_ = symbol("Js");