Function integrate

integrate(f: (x: number) => number, a: number, b: number, Ndx?: number): number
Compute the definite integral of a function.
Parameters
- f: (x: number) => number
  Function f(x)
- a: number
  The miminum integral bound
- b: number
  The maximum integral bound
- Ndx: number = 1e3
  The number of rectangles to compute
Returns number
F(b)-F(a)
Example
```
const y = SMath.integrate(x => 3 * x ** 2, 1, 2); // 7
```