Active_pos or Active_rot is not an attribute of brax_sys in version 0.0.13? Is some other version containing it?

Hi~
I am currently following a physics-informed work repo [Physics-informed nn](https://github.com/wuwushrek/physics_constrained_nn/issues/1) which use brax as a benchmark. In there they use `active_pos` and 'active_rot' to remove the inactive state dimension from the total state like this:
```def index_active_posrot(brax_sys):
""" This function computes the indexes of the states of the system that are active during the simulation
:param brax_sys : A brax object of class System representing a dynamical system
"""
pos_indx, quat_indx, rot_indx = [], [], []
pos_indx = brax_sys.active_pos.astype(bool)
rot_indx = brax_sys.active_rot.astype(bool)
quat_indx = jnp.sum(brax_sys.active_rot, axis=1) >= 1
quat_indx = jnp.hstack((quat_indx.reshape(-1,1), brax_sys.active_rot.astype(bool)))
return pos_indx, quat_indx, rot_indx
```

But I CANNOT find the `active_pos` and 'active_rot' as an attribute of brax_system？ Could it be the problem of brax version? My brax version is `0.0.13`, and which version should I choose? And where can I found the source code of the corresponding version? Could you help us please?