'integral' collapse does not work for grouped collapse 

This should work, rather than saying measure must be True when it has been set thus:
```
>>> import cf
>>> cf.__version__
3.4.0
>>> f = cf.example_field(2)     
>>> g = f.collapse('T: mean', group=cf.Y(), weights=True, measure=True)
ValueError: Must set measure=True for 'integral' collapses
```

This bug will be fixed for 3.5.0