cellsize attribute does not work for coordinates with more than 1 dimension

Currently:
```
>>> c
<CF AuxiliaryCoordinate: time(4, 9) days since 1970-01-01 00:00:00 >
>>> c.cellsize
<CF Data(4, 2): [[1.0, ..., 1.0]] d>
```

We should get:

```
>>> c.cellsize
<CF Data(4, 9): [[1.0, ..., 1.0]] d>
```