Maintain lazy bounds in Aux Factory derivation

[DPeterK]

If you print a cube with a bounded derived coordinate it realises the lazy bounds of the coordinates from which the derived coordinate is derived: 😱

>>> cube = iris.load_cube(iris.sample_data_path('hybrid_height.nc'))
>>> aux_coord = cube.coord('sigma')
>>> print aux_coord.has_lazy_points()
True
>>> print aux_coord.has_lazy_bounds()
True

>>> print cube

>>> print aux_coord.has_lazy_points()
True
>>> print aux_coord.has_lazy_bounds()
False

More accurately, printing a cube with a derived (bounded) coordinate triggers the deferred construction of the derived coordinate. If this derived coordinate is bounded then an oversight in the code that calculates the derived coordinate from the dependency coordinates, such as sigma in the example above, would cause the bounds to be accessed directly and so realise the bounds.

[corinnebosley]

This needs a test. Does it also need a What's New entry?

[bjlittle]

Closed by #2590