Ordered Init/Step/Exit/ExitCondition Functions

[ptheywood]

Init/Step/Exit/ExitCondition functions are all (mostly?) stored in unordered data structures, so the execution order of them is undefined.

Previous decisision was to just document this (#270).

It would be useful in many models for the order to be defined and controllable by the user.

This could be done in several ways:

1. Ordered data structures, where the order is handlede by insertion order during model definitions
2. Ordered data structure, combined with layer / dependency options

Insertion order change will be much less effort, but providing a non-insertion-order based approach might be useful in some edge cases, although this would be achievable if required by providing a single init function, which internally calls other local functions in whichever order is desired.

There may also be some scope to consider how init functions interact with file io. (currently io occurs earyl before init functions, anything prior to this is done outside of the simulation execution phase.