Separate the robot from the environment

[diegoferigo]

In the current architecture, environment and robot are treated as a single entity since they are described together in the sdf file. We already have two different files for the two, and the world file exploits sdf capability to import elements from another sdf file.

Referring to #12, we have few robot-related singletons in our architecture, and it affects exploiting multiple threads for parallel simulations.

Properly handling the robot and the environment separately also in gympp::IgnitionEnvironment might simplify the support of multithread. Instead of relying on sdformat to combine world and model files, gym-ignition can do it directly manipulating the individually deserialized sdf files.

[diegoferigo]

cc @traversaro