Clarify Work Plan instantiation and distribution semantics.

The WORK_PLAN.top_level_plans attribute should be changed to List<TASK_PLAN>. WORK_PLAN.plans should potentially be converted to a function that returns the same result.

LINKED_PLAN.target should also be changed to a direct reference.

These changes will allow archetype slots / refs to be used at the definition level.

The relationship between WORK_PLANs, TASK_PLANs and where they are committed to the EHR when materialised is not clear in the current spec.

The relationship between where WORK_PLANs are executed with respect to enterprises and workers in the real world is also not clear - how is a Plan that clinical implicates N enterprises (say GP, multiple hospitals, specialists etc) coordinated if there is no single shared place to execute it?