Default-attributes are exported to yaml

[danielhuppmann]

When initializing a VariableCode, the attributes region-aggregation and skip-region-aggregation are automatically added and set to "False". When then exporting the CodeList to yaml, these attributes are written explicitly to the file, leading to very cumbersome, unwieldy files, see for example iiasa/ariadne-intern-workflow#42

This is implemented in this line (and following)

nomenclature/nomenclature/code.py

Line 142 in 0b3dec1

region_aggregation: Optional[List[Dict[str, Dict]]] = Field(

I see two options:

1. don't automatically add these fields to the VariableCode if they are not defined in the source yaml/xlsx file
2. remove these attributes (if False) when exporting to yaml

[phackstock]

Thanks for opening this issue @danielhuppmann.
I have been aware of it and would strongly suggest option 2.
Reason being that it's easier and more logical to work with the values actually set to true or false. I think using none could be ambiguous.
The reason why I didn't fix it in the past was that I didn't find a quick way to check with pydantic if the value that a field has is the default value. But I'm sure there's a way.

[phackstock]

Took a quick look at the pydantic docs (https://docs.pydantic.dev/usage/models/#model-properties) and found a solution in form of the __fields_set__ attribute. It does everything we need.