Refactoring of the Interactions creation

Description

New framework to create and initialize interactions:

• There is now an interactions factory that given a system and a list of partners creates all compatible interactions between the system and the partners. For this to work, each system and each interaction partner needs to know which interactions they support. This is stored as a list.
• Now all systems are connected by ghost interactions, regardless if there are or not other interactions connecting the systems.
• Replaced the add_interaction_partner system method by a init_interactions method and removed the has_interaction system method, as it is not needed anymore.

News snippet

New framework to create and initialize interactions.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.