`example_fields()` should return a `Fieldlist` not a regular list

Unlike `cfdm` which has no concept of a `Fieldlist`, cf-python should return fields contained in a `FieldList` object rather than a regular Python list, which it returns at the moment:

```python
>>> type(cfdm.example_fields())  # expect a list
<class 'list'>
>>> type(cf.example_fields())  # expect a FieldList
<class 'list'>
```

