Select last reference cube.

This PR corrects an oversight.

Normally, after merging there should only be one reference cube e.g. orography for hybrid height, but there can be instances when the data-feed may contain multiple versions.

The PR allows such a data-feed to be loaded, an appropriate warning message issued and the last reference cube selected by default.

By setting `unique=False` merge will return multiple cubes when constructing the higher dimensionality and there are more than one candidate cube that wants to occupy a specific index in the higher dimensionality space. 

If the default merge `unique=True` behaviour is maintained, then an `iris.exceptions.DuplicateDataError` will be raised, and the data-feed cannot be loaded by the user.