Split setuptools packaging of gym-ignition and scenario

[diegoferigo]

It is a while I have this in the back of my mind. For long time now, the pure-Python gym-ignition package and the hybrid C++/Python ScenarIO package are completely independent and can happily live in different repositories. However, from the maintenance point of view, this did not and does not really work well for us.

Another option is having two different setup.py files, one for gym-ignition and another for scenario. After all, the existing gym-ignition package already installs these two folders inside site-package, and it is not a really common pattern.

From the users point of view, nothing really change. The after-split gym-ignition package will depend on the new scenario package, and if anyone from downstream that has gym-ignition in their setup.py file, they will transitively get also the new scenario package.

From our point of view, instead, we should only do minor modifications to our CI/CD pipeline.

From the superbuild point of view (cc @traversaro) there's nothing to change. The CMake project will not be touched.

[diegoferigo]

Closed via #346