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.
It would be clearer but it also does not work because `data` looks like:
```python
data = [np.array([array, array, array]), array([array, array, array])]
```
So I needed that double level comprehension to get around the two separate arrays within the list.

(For reference, trying `data[:, :, 0, 0]` caused a `TypeError: list indices must be integers, not tuple`.)