Allow simple customization of the action and observation space

[benedikt-schesch]

[benedikt-schesch]

@BDonnot Do you think this doesn't cause a problem on the lower layers? If you think it's good, I would also modify the docs to mention this before merging.

[BDonnot]

Hello,

A few remarks remarks here (before a more in depth review)

Can you make your pr to dev_1.9.7 branch instead?

And also if tests do not pass then it cannot be merged. Do you have an idea on the failing tests?