external_variables missing from CMIP6_required_global_attributes

[durack1]

Following PCMDI/cmor#541 and PCMDI/cmip6-cmor-tables#278 external_variables will need to be added to the CMIP6_required_global_attributes.json

@mauzey1 @taylor13

[taylor13]

external_variables is only conditionally required. It is required when areacella and/or areacello and/or volcella and/or volcello are included in the cell_measures attribute. Can PrePARE do a conditional test?

In any case I think but am not sure that "required_global_attributes" includes only those that are invariably require, not conditionally required.

[durack1]

@taylor13 good point, however I think this will need to be solved downstream as it was an omission that needed updating. @mauzey1 let me know if this is an incorrect assumption

[mauzey1]

@taylor13 @durack1
Okay, I will instead make a test in PrePARE that will check for external_variables if values are present in cell_measures.

Including external_variables in required_global_attributes was causing issues with other tests so we should revert that change.

[durack1]

@mauzey1 thanks for the heads up, I had not realized how intricate this was and so will revert #849

[durack1]

Reopening as external_variables is a conditional attribute, and consequently is causing problems when defined as required