Add base_frame_id param (defaults to base_link)

The `nav_msgs/Odometry` message specifies the `child_frame_id` field, which was previously not set.

This PR creates a parameter to replace the previously hard-coded value of the `child_frame_id` of the published `tf` frame, and uses it in the `odom` message as well.

Opened against `hydro-devel` because this change is minor. It does change the default value of a field, though, which is noteworthy.