Function relation

relation(a: number | Geometry.Point, b?: number): PointRelation
Tracks the relation between two points.
1. Call Points.relation with the initial reference point
2. You get back a function
3. Call the function with a new point to compute relational information.
It computes angle, average, centroid, distance and speed.
```
import { Points } from "https://unpkg.com/ixfx/dist/geometry.js";

// Reference point: 50,50
const t = Points.relation({x:50,y:50}); // t is a function

// Invoke the returned function with a point
const relation = t({ x:0, y:0 }); // Juicy relational data
```
Or with destructuring:
```
const { angle, distanceFromStart, distanceFromLast, average, centroid, speed } = t({ x:0,y:0 });
```
x & y coordinates can also be used as parameters:
```
const t = Points.relation(50, 50);
const result = t(0, 0);
// result.speed, result.angle ...
```
Note that intermediate values are not stored. It keeps the initial and most-recent point. If you want to compute something over a set of prior points, you may want to use Trackers.points
Parameters
- a: number | Geometry.Point
  Initial point, or x value
- Optionalb: number
  y value, if first option is a number.
Returns PointRelation