New features: you can add several rigid bodies, several markers and create new rigid_bodies

[Juancams]

Hi!!

As the title says, I have added the following:

• Renamed link_name to rigid_link to add a rigid_body.
• If you add multiple rigid_links, multiple rigid_bodies are created in different links.
• You can add marker_link in your plugin to add a marker on that link.
• Added service to group several markers in a rigid_body. The centroid of the polygon they form will be taken and the orientation will be given by the link that is specified.

In the future, I would like to add an rqt plugin that can add new rigid, markers, and group markers in rigis, as well as being able to do it from mocap4r2cli.

[aaggj]

It looks good to me, merged