Add "item" keyword to construct selection methods

Add an `item` keyword to `construct`, `dimension_coordinate`, and similar methods that will return both a unique construct and its identifier in an efficient manner:

```python
>>> import cf
>>> f = cf.example_field(1)
>>> f.construct('latitude', item=True)
('auxiliarycoordinate0',` <CF AuxiliaryCoordinate: latitude(10, 9) degrees_N>)
```

