Function timerWithFunction

timerWithFunction(
    fn: (v: number) => number,
    timer: CompletionTimer,
): HasCompletion & Timer & { get isDone(): boolean } & {
    compute: () => number;
}
Wraps timer, computing a value based on its elapsed value. fn creates this value.
```
const t = timerWithFunction(v=>v/2, relativeTimer(1000));
t.compute();
```
In the above case, relativeTimer(1000) creates a timer that goes from 0..1 over one second. fn will divide that value by 2, so t.compute() will yield values 0..0.5.
Parameters
- fn: (v: number) => number
- timer: CompletionTimer
Returns HasCompletion & Timer & { get isDone(): boolean } & { compute: () => number }
- getisDone():boolean
  Returns true if this timer has completed.
- compute: () => number