Split scenario and gym-ignition Python packages

[diegoferigo]

Closes #344. After this change, it should be more clear that gym-ignition is meant for RL applications, and scenario for a more broad use that is not related to RL. Those interested in just scenario can avoid to install also gym-ignition.

• Created new package (with setup.py) in the scenario folder, former cpp.
• gym-ignition pure Python package now depends from scenario.
• Update Docker images (currently only used in the website pipeline).

CI/CD pipeline changes:

• Split workflow in multiple jobs
• The workflow runs on default runners, it will be easier to add multiplatform support in the future
• The colcon workspace (nightly channel) is built only once and it gets cached even if there are build or test errors
• The new Build and Test job can be integrated more easily with conda (Test conda installation #279, Add workflow using conda to install dependencies #313)
• Create scenario wheels for both Python 3.8 and 3.9
• New complete artifact archive with both sdist and wheels for scenario and gym-ignition

TODO:

• Finalize new scenario README that will be included in the new PyPI package
• Update installation instructions
• Clean docker images
• Fix website workflow
• Claim the scenario PyPI package (PEP 541 Request: scenario pypi/support#1082) Using scenar-io while waiting.
• Check if the PyPI token works for both scenario and gym-ignition, otherwise generate a new one

[diegoferigo]

Waiting the decision on the PyPI name in the dedicated issue, maybe we can start moving this forward with the scenar-io name. The import will remain the same (scenario), it's just the PyPI package that will have a temporarily name. I think until everything remains in devel this should be fine.