The "select" keyword of cf.read does not work with PP and UM files

```
>>> import cf
>>> cf.read('/home/david/test2.pp').select('eastward_wind')
[<CF Field: eastward_wind(time(3), air_pressure(5), grid_latitude(110), grid_longitude(106)) m s-1>]
>>> cf.read('/home/david/test2.pp', select='eastward_wind')
[]
```

