add short_name for variables

[ThibHlln]

While CF standard names should be retained because they reduce the ambiguity in what the variables are about, in the code, this makes for very long names which impedes on readability. So alongside standard_name, it would be good to have short_name using those in the AMIP column in the CF standard name list if they exist, or with new ones (maybe look at ISIMIP). At the moment, the contributors are likely to bind the argument names (CF names) to new bespoke shorter names in the code.