Troubles with orphan_variables when running create_ping_files.py

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Hi Stephane,

I try to get create_ping_files.py running. I am only able to achieve this by commenting in dr2xml.py the following lines:

         if context in settings['orphan_variables'] and \
            v.label in settings['orphan_variables'][context] :
             lvars.append(v)

This is probably not the correct solution. I have no idea what the consequences are of this change? My goal is to use dr2xml for CMIP6 data requests. The modified create_ping_files.py which I am using can be found here.