Distribute through conda-forge

[diegoferigo]

This issue tracks the progress of distributing ScenarIO and gym-ignition through conda-forge. Previous experiments done in #279.

• [x] https://github.com/conda-forge/libignition-gazebo-feedstock/pull/25
• [x] https://github.com/ignition-tooling/release-tools/issues/459
• [x] https://github.com/conda-forge/staged-recipes/pull/16572
• [x] https://github.com/conda-forge/staged-recipes/pull/16573
• [x] https://github.com/robotology/idyntree/pull/922
• [x] https://github.com/conda-forge/idyntree-feedstock/pull/8
• [x] https://github.com/robotology/gym-ignition/pull/406
• [ ] https://github.com/robotology/gym-ignition/pull/419
• [ ] https://github.com/conda-forge/staged-recipes/pull/16582
• [ ] TODO: open PR for gazebo-scenario-plugins

The project can then be installed as follows, without the need to install manually Ignition in the system:

mamba create -n gym_ignition_env "python==3.8.*" mamba
conda activate gym_ignition_env

# Install gym-ignition...
mamba install gym-ignition

# ... or only the ScenarIO Python bindings...
mamba install scenariopy

# ... or only the ScenarIO C++ library.
mamba install libscenario