d3.thresholdScott returns NaN for single-element arrays

E.g., 

```js
d3.thresholdScott([1], 0, 1)
```

It should probably return 1 in this case instead (which matches d3.thresholdSturges).

Also… this returns infinity, which should probably also be fixed.

```js
d3.thresholdFreedmanDiaconis([1], 0, 1)
```