Several further improvements to the lists in the context of multisystem calculations

Description

Changed the way how we iterate over list to make it both simpler and to allow for multiple nested loops over list elements. Changed the propagator class to be an extension of the linked list. Also added a wrapper list of interactions to later avoid some select type statements.

News snippet

Several further improvements to the list in the context of multisystem calculations.

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.