Refactoring of the interaction classes in the multisystem framework

Description

Some refactor of the interaction base classes in the multisystem framework. The main purpose is to make the code easier to understand. Main changes:

1. moved all components of interaction_t to interaction_with_partner_t
2. renamed interaction_t to interaction_surrogate_t
3. renamed interaction_partner_t to interaction_t

The end result is an interaction_t class that includes all the components and an "empty" surrogate class used only to avoid circular dependencies.

Also added as much documentation as I could.

News snippet

Some refactor of the interaction base classes in the multisystem framework.

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.