Add control values to keyframes.

- Add basic test for keyframes.
- Add missing documentation for keyframe mocap positions and quaternions.

PiperOrigin-RevId: 459021649
Change-Id: I91cf7ecbddc6262e8c72a868eeb82d627f389fb3

src/user/user_model.cc

@@ -1935,6 +1935,8 @@ void mjCModel::CopyObjects(mjModel* m) {
     for (j=0; j<nmocap; j++) {
       mju_normalize4(m->key_mquat+i*4*nmocap+4*j);
     }
+
+    copyvec(m->key_ctrl+i*nu, keys[i]->ctrl.data(), nu);
   }
 
   // save qpos0 in user model (to recognize changed key_qpos in write)
@@ -2878,6 +2880,9 @@ bool mjCModel::CopyBack(const mjModel* m) {
       copyvec(pk->mpos.data(), m->key_mpos + i*3*nmocap, 3*nmocap);
       copyvec(pk->mquat.data(), m->key_mquat + i*4*nmocap, 4*nmocap);
     }
+    if (nu) {
+      copyvec(pk->ctrl.data(), m->key_ctrl + i*nu, nu);
+    }
   }
 
   return true;