Oh no ..... :fire:

Auxiliary coordinates require the same treatment as coordinates @cpelley ... tsk, tsk, tsk!

```
>>> import iris
>>> import numpy as np
>>> c1 = iris.cube.Cube(np.arange(10), 'air_temperature')
>>> c2 = iris.cube.Cube(np.arange(10), 'air_temperature')
>>> c1.add_aux_coord(iris.coords.AuxCoord(np.arange(10), 'time'), 0)
>>> c2.add_aux_coord(iris.coords.AuxCoord(np.arange(10, 20), 'time'), 0)
>>> print c1
air_temperature / unknown           (*ANONYMOUS*: 10)
     Auxiliary coordinates:
          time                                  x
>>> print c2
air_temperature / unknown           (*ANONYMOUS*: 10)
     Auxiliary coordinates:
          time                                  x
>>> print c1.coord('time')
AuxCoord(array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]), standard_name='time', units=Unit('1'))
>>> print c2.coord('time')
AuxCoord(array([10, 11, 12, 13, 14, 15, 16, 17, 18, 19]), standard_name='time', units=Unit('1'))
>>> clist = iris.cube.CubeList([c1, c2])
>>> iris.save(clist, 'blimey.nc')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/net/home/h05/itwl/projects/git/iris/lib/iris/io/__init__.py", line 382, in save
    saver(source, target, **kwargs)
  File "/net/home/h05/itwl/projects/git/iris/lib/iris/fileformats/netcdf.py", line 770, in save
    dataset = netCDF4.Dataset(filename, mode='w', format=netcdf_format)
  File "netCDF4.pyx", line 1375, in netCDF4.Dataset.__init__ (netCDF4.c:16538)
RuntimeError: NetCDF: Can't add HDF5 file metadata
```

While you're fixing this, please pause for thought and consider ```_create_cf_grid_mapping``` you might also find a similar :bomb: ...