Multiplexed aux publisher

[alaurenzi]

On branch aux_publisher I have written a draft of an aux field publisher for ROS, implemented as an RT plugin. The logic is the following:

• through a config file, the user specifies the list of fields to be published in a multiplexed fashion. By default, all fields are published.
• aux values are published each to its own topic, such as /xbotcore/aux/iq_ref, etc

Have a look to the README

The code assumes that the method get_xbotcore_joint()->get_op_idx_ack(jid, aux_id) will return the code of the aux field contained in the same PDO frame.

@liesrock