Index.getIndexString parameter definition missed in v0.9.0

"Expected 0 arguments, but got 2." error occur when I try to run a sample in TSX as follows.

```
const requestsSeries = new TimeSeries({
name: 'requests',
columns: ['time', 'requests'],
points: requests.map(([d, value]) => [Index.getIndexString('1h', new Date(d)), value]),,
})
```

I found current definition is different with real function parameter.

Current wrong definition:
[pond/src/index.d.ts](https://github.com/esnet/pond/blob/v0.9.0/src/index.d.ts#L223)
```
static getIndexString(): string;
```
Real function parameter:
[pond/src/pond/lib/index.js](https://github.com/esnet/pond/blob/v0.9.0/src/pond/lib/index.js#L101)
```
static getIndexString(win, date) {
```

Please revise it as follows:
```
static getIndexString(win: string, date: Date): string;
```