Function frequencyTimer

frequencyTimer(frequency: number, options?: Partial<TimerOpts>): ModulationTimer
A timer based on frequency: cycles per unit of time. These timers return a number from 0..1 indicating position with a cycle.

In practice, timers are used to 'drive' something like an Oscillator.

By default it uses elapsed clock time as a basis for frequency. ie., cycles per second.

It returns a ModulationTimer, which allows for a modulation amount to be continually applied to the calculation of the 'position' within a cycle.
Parameters
- frequency: number
  Cycles
- options: Partial<TimerOpts> = {}
  Options for timer
Returns ModulationTimer
Example: Prints around 0/0.5 each second, as timer is half a cycle per second
```
import { frequencyTimer } from "https://unpkg.com/ixfx/dist/flow.js"
const t = frequencyTimer(0.5);
setInterval(() => {
 console.log(t.elapsed);
}, 1000);
```