Get the most recent lean body mass. The value is a number representing the weight in pounds (lbs)

On success, the callback function will be provided with a `leanBodyMass` object containing the leanBodyMass `value`, and the `startDate` and `endDate` of the sample. *Note: startDate and endDate will be the same as leanBodyMass samples are saved at a specific point in time.*

```javascript
 AppleHealthkit.getLatestLeanBodyMass(null, (err: Object, results: Object) => {
  if (err) {
    return;
  }
  console.log(results)
});
```

```javascript
{
	value: 176,
	startDate: '2016-07-08T12:00:00.000-0400',
	endDate: '2016-07-08T12:00:00.000-0400'
}
```