franka_ros: [bug] Gazebo gripper control is broken

In the current version, when working with the gazebo simulation, the gripper width that is sent to the /panda/franka_gripper/gripper_action/goal topic is not handled correctly. The problem is caused by the following code:

https://github.com/frankaemika/franka_ros/blob/054af531f43f3fc1ddb439cc4102a41f87a8e0da/franka_gazebo/src/franka_gripper_sim.cpp#L436

Due to this, the desired gripper with now becomes 0 when the gripper width is smaller than the current gripper width and 0.08 when it is bigger than the current gripper width. Consequently, users can now only open/close the gripper.

I created #173 to address this problem.

Steps to reproduce

1. Follow the documentation to install libfranka library.
2. Create a new catkin workspace.
3. Clone franka_ros package in this workspace.
4. Clone https://github.com/rickstaa/panda_moveit_config/tree/adds_gazebo_simulation in this workspace.
5. Build the workspace following the steps explained in the documentation.
6. Start the Panda simulation using the roslaunch panda_moveit_config demo_gazebo.launch.
7. Set the ros namespace `export ROS_NAMESPACE=/panda
8. Run the script below to send a MoveIt gripper command.
9. See that you can not set the gripper to a specific width, since it will only close and open.

"""Small script to see that the gazebo GripperCommand action does not set the gripper
width.
"""

import sys

import moveit_commander
import moveit_msgs.msg
import rospy

GRIPPER_WIDTH = 0.08

if __name__ == "__main__":

    # Initiate MoveIt commander and node
    moveit_commander.roscpp_initialize(sys.argv)
    rospy.init_node("test_gripper_width", anonymous=True)

    # Create commanders
    robot = moveit_commander.RobotCommander()
    scene = moveit_commander.PlanningSceneInterface()

    # Load arm and hand groups
    hand_move_group = moveit_commander.MoveGroupCommander("hand")

    # Display the trajectory
    display_trajectory_publisher = rospy.Publisher(
        "/move_group/display_planned_path",
        moveit_msgs.msg.DisplayTrajectory,
        queue_size=20,
    )

    # Send the gripper width command
    hand_move_group.set_joint_value_target([GRIPPER_WIDTH/2, GRIPPER_WIDTH/2])
    (hand_plan_retval, plan, _, error_code) = hand_move_group.plan()
    retval = hand_move_group.execute(plan, wait=True)

    # Calling ``stop()`` ensures that there is no residual movement
    hand_move_group.stop()