`Data._axes` is not updated after a `cf.Field.collapse`

Data._axes is not updated after a cf.Field.collapse. If squeeze=False (the default) then this is not a problem, but if squeeze=True then the removed collapsed axes need stripping from returned Data object _axes attributes.

>>> # Current
>>> import cf
>>> f = cf.example_field(0)
>>> g = f.collapse('X: mean', squeeze=True)
>>> g.data._axes
('dim0', 'dim1')  # WRONG

>>> # Should be ...
>>> import cf
>>> f = cf.example_field(0)
>>> g = f.collapse('X: mean', squeeze=True)
>>> g.data._axes
('dim0',)  # RIGHT