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.
Here's fun...  I don't think we need this property at all any more : Everything goes via lazy_data.
If I comment it out, the tests still all work.

(N.B. just like the newer ```grib._message._GribMessage```)