Add support for nested models

[srmainwaring]

The original Gazebo version of the plugin allowed models to be nested, however this is not currently supported in the Ignition version.

Motivation

It is useful to be able to create variants of a basic model that are equipped with different sensors or other peripherals. The base robot, each peripheral, and other components can each be defined as separate models which may then be composed into new robot variants.

For example: a base rover defines the chassis, wheel train and minimal sensor array (viz. IMU sensor). A peripheral model defines a laser scanner. A variant of the rover is then defined by composing the base rover and the laser scanner.

Issues

For composition to work, a plugin must be able to locate links, joints and sensors in composite models, and also define and resolve topics which have been scoped with the nested model names. The following list of issues have been identified:

1. A nested IMU sensor cannot be resolved: The code in ArduPilotPlugin::PreUpdate that uses the entity component manager (ECM) to search for a named IMU component is not resolving IMU sensors in nested models.

2. Nested joints cannot be resolved: The function ignition::gazebo::Model::JointByName only searches for joints that have the top level model as a parent. There is no recursion down into nested models.

3. Topics: the rules for defining topics have changed between classic Gazebo and Ignition. In particular the prefix ~/ which used to be resolved to a namespace prefix comprising the world and top level model name is no longer permitted. The ArduPilot plugin needs to subscribe to a number of topics published by sensors and these need to reference topics published by sensors in the top level model as well as those in nested models. Furthermore the plugin should only need to specify topics up to a world and model namespace which is automatically resolved, otherwise every additional instance of a model spawned will need to be edited to ensure the correct sensor topics are subscribed to - this is not practical if say you are attempting to create a swarm of robots.