Correct iris faulty generation of altitude bounds when reading files with hybrid height coordinate

[matthew-mizielinski]

There is a long running discussion around whether the bounds on the vertical coordinate are being appropriately set on CMORised data; PCMDI/cmor#177 and SciTools/iris#3678.

At present any iris v2 code will give spurious bounds as it won't read in the b_bounds (a.k.a. sigma bounds) due to the lack of variable attributes. This leads to invalid altitude bounds over orography as the point values are used instead.