Allow importing gym_ignition even if Ignition Gazebo is not installed in the system

[diegoferigo]

This is more or less similar to what we were doing long time ago with pybullet (#110, #149).

In few words, after #346 the pure-Python gym-ignition package will depend on scenario. When gym-ignition gets installed in a system (currently only Ubuntu is supported), the wheel of scenario will be installed first. Since the wheel of scenario is not self-contained (i.e. it needs to find in the system the Ignition libraries), if Ignition is not installed, the import of scenario.bindings.gazebo will fail. Therefore, in such systems, gym-ignition cannot be imported.

This seems quite logical, however there are use cases in which resources that are currently included in gym-ignition could be useful even if Ignition is not available in the system. For instance, all the high-level iDynTree classes.

Importing lazily all the scenario.bindings.core is quite easy to do, and I think we should try to support this use case.

cc @GiulioRomualdi @paolo-viceconte @traversaro

[diegoferigo]

This does not play that well with the current status of type hinting. Putting it in the backlog for the moment.