move canopen_chain_node service definitions into can_msgs

[jschleicher]

Inside canopen_chain_node there are two service definitions: GetObject.srv and SetObject.srv. To allow other ROS nodes to <depend> on slim packages (instead of depending on canopen_chain_node), it would be cleaner to have all service and message definitions of the meta-package inside can_msgs.

What do you think?

[mathias-luedtke]

These two services are quite specific to canopen_chain_node (and canopen_master). That's why I kept them in the package.

If there is a need for the service definitions only, I would accept a pull request. However, they do not belong in can_msgs, but in a more specialized package (e.g., canopen_msgs).