Updated sample data NetCDF files.

Updated the NetCDF files produced from the Met Office's data zoo.

These changes have come about through changes to Iris, rather than changes to the data themselves. Most notably, the  sample_data/A1B_north_america.nc and  sample_data/E1_north_america.nc have changed their dimensions to be ```time, lat, lon``` rather than ```forecast_period, lat, lon```.