realm vs. modeling_realm

[taylor13]

@mauzey1 @matthew-mizielinski @JamesAnstey @durack1: In the CMOR variable tables, the "realm" can be set in the header and "modeling_realm" can be set for an individual variable. As I vaguely recall, CMOR writes "realm" as a global attribute in files unless "modeling_realm" is found, in which case it writes "modeling_realm". Either of these can be a list of realms (i.e., multiple realms may be listed).

Since the global attribute is named "realm", I think it would be less confusing if in the CMOR table files all instances of "modeling_realm" were replaced by "realm". I suppose old cmor tables would not be readable, but should we make this change, or wait for CMIP8?

[matthew-mizielinski]

While this change makes things neater it is a breaking change, which would need to be flagged as such, so I'd put this in the hopper for CMOR version 4 and defer work until a later date.

[durack1]

I tend to agree, neat vs working, the latter is the focus now, so I will add this as a CMOR4 and we can park discussions for post-CMIP7