Function pingPong

pingPong(
    interval: number,
    lower: number,
    upper: number,
    start?: number,
    rounding?: number,
): Generator<number, never, unknown>
Ping-pongs continually back and forth a lower and upper value (both inclusive) by a given interval. Use pingPongPercent for 0-1 ping-ponging

In a loop:
```
for (const c of pingPong(10, 0, 100)) {
 // 0, 10, 20 .. 100, 90, 80, 70 ...
}
```
Manual:
```
const pp = pingPong(10, 0, 100);
let v = pp.next().value; // Call .next().value whenever a new value is needed
```
Parameters
- interval: number
  Amount to increment by. Use negative numbers to start counting down
- lower: number
  Lower bound (inclusive)
- upper: number
  Upper bound (inclusive, must be greater than start)
- Optionalstart: number
  Starting point within bounds (defaults to lower)
- Optionalrounding: number
  Rounding is off by default. Use say 1000 if interval is a fractional amount to avoid rounding errors.
Returns Generator<number, never, unknown>