Function gaussian

gaussian(standardDeviation?: number): (t: number) => number
Returns a roughly gaussian easing function
```
import { Easings } from "https://unpkg.com/ixfx/dist/modulation.js";
const fn = Easings.gaussian();
```
Try different positive and negative values for stdDev to pinch or flatten the bell shape.
Parameters
- standardDeviation: number = 0.4
Returns (t: number) => number