Melodic migration

See merge request !1

description.launch: loads the robot_description parameter and launches the robot_state_publisher node.

wheel_static_transf.launch: publishes four static transform for the four wheels of the robot.

helena_description: platform.gazebo: odometry publisher on base_link frame (same as real robot Rosaria driver does)

 - helena.xacro. camera rotated pi/2 on roll (real camera set up)
 - base_footprint hangs from base_link frame

Removed an STL file.
Adjusted the position of the sensors to coincide with the ones on the robot.

Added all sensors (front and rear hokuyo, laser_3d and cameras).
Added the models and urdf parts for the laser 3D.
Added the calibration file for the new front camera.
Removed the base_footprint frame due to errors with the odometry.

 - added cmd_vel_mux and velocity_smoother node launchs
 - added tag detection files and launchs (arpose)
 - other launch updates

 - added machine=embedded to some launchs
 - added description/urdf/platform files (missing on last commit)

The wheeldiameter parameter must actually be the wheel radius.
Added a launch to teleoperate the simulated robot.

Added the gazebo simulation node for the helena robot.