Add param for vel scaling of the scan movement

[Chamango90]

• The new parameter uses setMaxVelocityScalingFactor() to set the scaling of the velocity for the movement to/between the scan points.

• The function is called with the parameter in the RobotScan::init() function. This is actually not required because of the next bullet-point. However, I left it to be consistent with the other params like e.g. move_group_ptr_->setMaxVelocityScalingFactor(params_.move_scan_vel_scaling);

• I also added this function to the RobotScan::create_scan_trajectory(...) function. So if one changes the parameter in the gui, it is applied for the next scan process. setEndEffectorLink(params_.tcp_frame)

• Parameter got added in the RobotScanParameters.msg

• I set the default value and the one in robot_scan.yaml(s) to 1, which applies no scaling

• Added the parameter to the gui(s)

• Had to increase the hight of the gui(s) to make space for the new field

[AustinDeric]

Looks great to me. Any objections? If not i am going to merge in 48 hours.