ERA5 hurs or huss derivation

[malininae]

I've noticed that ERA5 single levels doesn't have surface humidity, so to obtain the hurs or huss one would need to use input from tas, tdps and ps. What's the best way to derive it operationally? The only solution I cam up with is to create a derivation function in the preprocessor. Would it be a good idea? (I am working with daily data, so hurs/huss derivation will make sense climatologically). This issue is remotely related to #1806.