Add data attribute to FieldCollation

Adds a data attribute to `FieldCollation` so that the data in the input PPField iterator can be accessed by downstream consumers of `FieldCollation` objects.
> data looks like:
> 
> data = [np.array([array, array, array]), array([array, array, array])]

That's because you've done `ndarrays(collation.data)`, but `ndarrays()` expects a *sequence* of Array objects. What it should say instead is `collation.data.ndarray()`.