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.

changed milestone to %14.0

added Documentation Multisystem Refactoring labels

marked this merge request as ready

requested review from @martin.lueders

Trivial comment about the MR description. You probably changed the strategy during this MR. You are writing that you moved all components from interaction_t to interaction_with_partner_t, but it is the other way around (there is no more interaction_with_partner_t). Would you mind, updating the description, or are you still in the process, and will change the names of the resulting classes again?

changed the description

resolved all threads