Add a function that represents a system, e.g. Physics, Rendering, etc.
This needs to be in the structure: (fn [system delta]) where 'delta' is the number of milliseconds
since the last game tick. This will also need to return the system in the state you want passed to the
next system-fn, and ultimately out of process-one-game-tick.
This will then be called directly when `process-one-game-tick` is called

Same as `add-system-fn`, but will only execute the `system-fn` after `threshold` milliseconds has been equalled or passed.

Optional convenience function that calls each of the system functions that have been added in turn, with the provided delta.