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

[sadielbartholomew]

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:

>>> type(cfdm.example_fields())  # expect a list
<class 'list'>
>>> type(cf.example_fields())  # expect a FieldList
<class 'list'>