Remove external use of `_data` attribute.

The data API of the 2D field formats is currently a bit of a mess, with `_data` being accessed from outside the classes in order to provide to deferred loading. This PR attempts to improve matters by borrowing the `data`/`lazy_data()` API from the `Cube` class.
>>  I don't think we need this

After discussion, prefer to leave the 'fields' types as they are, to provide low-level access.
Eventually, they can probably all provide data/lazy_data as required.