Examples
>>> len(f)
6
>>> f
[<CF Field: eastward_wind(110, 106)>,
<CF Field: eastward_wind(110, 106)>,
<CF Field: eastward_wind(110, 106)>,
<CF Field: eastward_wind(110, 106)>,
<CF Field: eastward_wind(110, 106)>,
<CF Field: eastward_wind(110, 106)>]
>>> cf.aggregate(f)
>>> len(f)
1
>>> f
[<CF Field: eastward_wind(3, 2, 110, 106)>]