Add optional debug mode

[neo-alex]

🚀 Feature

Add an optional debug mode to scikit-decide.

Motivation

Currently, some (possibly silent) bugs can be difficult to find when creating a new domain (or solver).

Pitch

The scikit-decide library could add a global debug mode that the user might optionally activate (at the cost of a small overhead) to do a number of sanity checks automatically, e.g. asserting that all the actions/observations are indeed members of the declared action/observation spaces. Ideally, this feature should add zero overhead at runtime whenever the debug mode is not activated (this should be possible with the use of method decorators, which would simply return the original function when not in debug mode).