Replace raw Observer* by a shared ptr as part of world.

[MARIANOCEREDA]

Description

As mentioned in #860 , this aims to start removing raw pointers, and replace them by smart pointers.
In particular, this PR removes the Observer* ptr from world.hpp and replaces it with:

• A definition as a std::vector<std::shared_ptr<Observer>> in the World object, which owns the observers.
• A definition as a std::weak_ptr<Observer> in the ObserverHandle object, which accesses the object.

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
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