Refactor quantities

Description

This MR refactors the list of quantities (changing from a fixed size array to a linked list), and introduces the concept of parents of a quantity. This is necessary to ensure that a quantity is not updated beyond the iteration counter of the parent system, which occurred in some situations, e.g. when two systems interacted via on-demand quantities.

News snippet

Introduce linked lists of quantities and parents of a quantity to ensure quantities are not getting ahead of their parents.

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.