Function differentiate

differentiate(f: (x: number) => number, x: number, h?: number): number
Take the derivative of a function.
Parameters
- f: (x: number) => number
  Function f(x)
- x: number
  The x-value where to evaluate the derivative
- h: number = 1e-3
  Small step value
Returns number
f'(x)
Example
```
const y = SMath.differentiate(x => 3 * x ** 2, 2); // 12
```