Fix bugs in all examples

[hanson-hschang]

Make sure each example file is clear and runnable without any issue. That means:

• check comments if they are clear

• check the generated .blend file if the rendering is the same as expect

• camera_movement.py

• pose_demo.py

• projectile_motion.py

• pendulum.py

• elastica_timoshenko.py

• update single_rigid_rod_spring_action.py

• br2\br2_pushed.py with br2\callbacks.py