Add bed in URDF file of the simple robot and improve simulations

...t_1dof/launch/simple_robot_rviz_simulation_1dof_launch.py

...ot_1dof/simple_robot_1dof/forward_kinematics_node_1dof.py

ros2_ws/src/simple_robot_1dof/urdf/simple_robot.urdf

..._2dof/simple_robot_2dof/forward_kinematics_node_2dof_1.py

ros2_ws/src/simple_robot_2dof/urdf/simple_robot.urdf

..._3dof/simple_robot_3dof/forward_kinematics_node_3dof_1.py

..._3dof/simple_robot_3dof/forward_kinematics_node_3dof_2.py

ros2_ws/src/simple_robot_3dof/urdf/simple_robot.urdf