High-, mid- and low-level cloud fraction

[gnikulin]

I found that the standard HCLIM configuration for Euro-CORDEX saves high-, mid- and low-level cloud fraction (clh, clm, cll). These 3 variables are in the common CORDEX-CMIP6 output list (Tier2) and were included in the CMOR tables but it was decided to exclude them in Euro-CORDEX. I think we can also provide them in Euro-CORDEX since they have been saved. In addition, these variables are used for evaluation of HCLIM46 so can be useful in general. Priority is not so high. Description of the vertical coordinate is a bit different from other variables at pressure levels, e.g. in CORDEX-CMIP5

       double plev ;
		plev:axis = "Z" ;
		plev:bounds = "plev_bnds" ;
		plev:long_name = "pressure" ;
		plev:positive = "down" ;
		plev:standard_name = "air_pressure" ;
		plev:units = "Pa" ;
	double plev_bnds(bnds) ;
		plev_bnds:long_name = "Boundaries of the layer in which cloud area fraction is computed" ;
		plev_bnds:units = "Pa" ;