Map Extras from CKAN dataset and group across

See the TODO in lib/dms.js here https://github.com/datopian/frontend-v2/blob/master/utils/index.js#L83

Here's some sample code to do this ...

```javascript
function getExtrasAsObject(extras) {
  extras = extras || []
  return extras.reduce((newObj, x) => Object.assign(newObj, { [x.key]: x.value }), {})
}
module.exports.getExtrasAsObject = getExtrasAsObject
```