This singleton collects the callbacks used for all the agents. It is the first step toward the support of multiple agents (each of them associated to one of these callbacks) acting on independent environments.

Ignition plugins that need a gympp::Robot pointer will contain an IgnitionRobot object and they configure it with their sdf configuration.

The callbacks are registered by the plugins during their Configure step.

Temporarily, the declaration of the gympp plugin is done in the sdf. As soon as robot and environment will be split, IgnitionEnvironment will handle that again.

Now setupGazeboWorld has to ben called before setupIgnitionPlugin

This commit:

- Get the EnvironmentCallbacks pointer from the singleton. Plugins autoregister themselves during their Configure step.
- Split robot and world. They are stored in two different files and not anymore merged by sdformat. This allows parsing the model sdf and automatically get the models. The plugin is then attached to the model element, and this permits to the plugin to correctly initialize the gympp::Robot object.
- The plugin is loaded again from cpp instead of sdf.
- The pose of the model will be handled in another PR.