Extract fix

The modification/optimisation of the extract code path using nearest_neighbour_index means that:
```python
point = cube.coord('model_level_number').points[0]
cube.extract(model_level_number=point)
```
fails with ```ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()```. This is because ```__eq__``` for Cell is called and it cannot handle numpy floats or ints so the equality falls to the np.int which treats the cell as a tuple (i.e. an iterable) returning a numpy array e.g. ```array([False, False])``` which cannot be evaluated as a bool.

This PR fixes this bug allowing numpy ints and floats to be handled by ```Cell.__eq__()```

**This PR targets 1.5.x and is intended for a 1.5.1 release.**