Extract fix for Cell comparison

When doing:
```python
cube.extract(model_level_number=Cell(10))
```
an unexpected code path is followed as the Cell is an instance of collections.Iterable. It is therefore converted to a list (e.g. ```[10, None]```) and each cell in the coord is compared element-wise via ```call_func = lambda cell: cell in [10, None]```.

This PR fixes this so that cell equality is used.

**This PR targets 1.5.x and is intended for a 1.5.1 release.**