If the record's value isn't a simple conversion from the value in the feed
you can provide a `calculate` method. For example:

    {
        name: 'position',
        calculate: (record, field, data) => ({ lat: data.latitude, lng: data.longitude })
    }

- _record_ is the record instance
- _field_ is the field config
- _data_ is the value from the data feed

You can use `calculate` to convert a value from the feed, or to define a new record value.