Add motor xacro macro

[fjp]

I have added a xacro macro for motors in the include/common_macros.urdf.xacro.

If no mesh is provided in the new config/motor.yaml then a default box is used:

Other changes I have made (or that could be made):

• renamed material: material_dark_grey to dark_grey
• remove some whitespaces
• For the config/motor.yaml I removed the top level motor: like it is done in base.yaml and front_wheel.yaml. This makes the code shorter inside common_macros.urdf.xacro. Maybe this could be used for the other yaml files as well because the description is already in the dictionary name, e.g. motor_props, base_prop, wheel_props. I think having base_prop['base']... and wheel_props['wheel']... is a bit redundant. What do you think? This could be addressed in the future. Maybe there is a problem without the top level name?
• added a colored stl wheel file: wheel-colored.stl for diffbot, which should fix the color in Gazebo. Only now the color doesn't work in RViz (rims and tires will have the same color).
• TODOs
  • Fix the wheel placement of diffbot
  • Add sensors (camera, RPLidar A2, Ultrasonic, ...). Maybe we could have a mesh folder for common sensors that are usable for different robots.
  • Add documentation (nearly the same as for the wheel)

[fjp]

I reverted all commits (to avoid having two useless stl files in the git history) and closed this issue because I just saw that the colors/materials applied in Blender don't work in Gazebo. See the new PR #9