file format a_3, a_4 need channel dimension

[gerritholl]

Debug file format a_3, a_4 need channel dimension. Currently we have:

ncdump -h /group_workspaces/cems2/fiduceo/Data/FCDR/HIRS/v0.8pre2/debug/noaa19/2017/02/11/FIDUCEO_FCDR_L1C_HIRS4_NOAA19_20170211120907_20170211135106_DEBUG_v0.8pre2_fv2.0.0.nc

[snip]

        float a_3 ;
                a_3:_FillValue = NaNf ;
                a_3:units = "dimensionless" ;
                a_3:long_name = "IWCT emissivity correction assumed in calibration" ;
        float a_4 ;
                a_4:_FillValue = NaNf ;
                a_4:units = "watt / hertz / meter ** 2 / steradian" ;
                a_4:long_name = "Bias harmonisation term" ;

[snip]

which is not very useful.