Handling of optional global attributes

[taylor13]

I'm moving here the part of the discussion In WCRP-CMIP/cmip7-cmor-tables#129 regarding handling of the source attribute, which is optional in CMIP7 (with its value uncontrolled).

As I understand it, CMOR gets its value for source from the cmor-cvs.json file, and the user can't override the value in the cmor-cvs.json file (if I correctly understand WCRP-CMIP/cmip7-cmor-tables#129 (comment)). I suggest that CMOR should treat source as follows:

• by default, extract for source whatever is recorded as source in cmor-cvs.json and write it as the source global attribute. (I think that is what happens now.).
• Enable users to override the source value found in cmor-cvs.json by specifying a different value in the *_input.json file. More specifically, if in the user's input file (e.g., https://github.com/PCMDI/cmor/blob/main/Test/CMOR_input_example.json) the user specified a value (other than "") for source, then CMOR would override the value provided by the cmor-cvs.json file.

We might want to do something similar for institution and experiment.

[taylor13]

Just noting that this is just something to consider. If the correction of the CMIP7 cmor-cvs.json file "source" information is implemented as discussed in WCRP-CMIP/cmip7-cmor-tables#133, then I don't think this is a priority.