As discussed, how will this handle non-monotonic 1d coords with bounds? e.g.
```python
c = iris.coords.AuxCoord([1,2,4,3], bounds=[[0.5, 1.5], [1.5, 3.5], [3.5, 4.5], [2.5,3.5]], long_name='bob')
c.nearest_neighbour_index(3.5)
```
Current code will return the first value I believe. This code will raise an AssertionError.