Support loading/saving netCDF with multiple coordinate systems

[lbdreyer]

In CF-1.7 the use of the grid_mapping attribute was expanded to store multiple coordinate systems. See example 5.10
Ref (for background, if needed): trac ticket 70

We should handle reading and writing files of these types of files.

[lbdreyer]

Agreed to delay with til Iris 3, as it is no longer such a high priority for the user requesting this

[pp-mo]

Note the relevance of #3796.
It probably makes good sense to do this first.
However, as noted there, the feature described in #3796 can affect the implementation of this in some cases. So it makes sense to bear that in mind.