fixed problem with saving cubes vertical levels but missing orography

A change introduced between 1.6 and 1.7 (when we added support for Hybrid pressure) means that loading hybrid height data from PP (or anywhere else) without an orography and then saving it to PP will "unroll" the cube stripping out the vertical level information. This is because the PP save rule checks for the presence of an aux factory rather than the model level, sigma, and level_height coords. This is more flexible as it allows the dependency names (i.e. the formula terms) to have different names. Unfortunately a lot of data lacks the orography and that currently stops the aux factory being created on load. Consequently the vertical coords are not saved. This PR provides a simple (inelegant) fix. See https://github.com/esc24/iris/compare/missing_vertical for a more advanced solution that delegates responsibility to the factory and allows load and save without reference surfaces. 

Targets v1.7.x