optimiser parameter-server configuration pattern is incompatible with ROS2 parameters

[BrettRD]

The Optimiser base class expects to load an XML data structure containing all relevant parameters from the parameter server.

In ROS2, all parameters must be forward-declared, making a dynamic tree extremely verbose, with a lot of data duplication to facilitate the necessary forward declarations.

An alternative is to load the old XML data structure via a retained message on a configuration topic, similar to how robot_state_publisher passes a URDF file to downstream entities. This opens the possibility of integrating fuse configurations into URDF models.

[BrettRD]

under ROS1, sensor_model, motion_model and publisher expect to load parameters from the root of the parameter server.
In ROS2, the parameter server is strictly name-spaced per node, meaning that sensor_model config needs to be listed against the node name of the sensor_model, not the node name of the optimizer

The human-readable names of plugins needs to be declared first

The resulting config files aren't much different, just more explicit about nodes and somewhat flatter.

[BrettRD]

proposed config layout in 4e4d80b
broader issue tracked in locusrobotics#275
ignoring the URDF possibility for now