Extend categorizations via definition file

[mikapfl]

Is your feature request related to a problem? Please describe.

At the moment, categorizations can only be extended programmatically, and stored only in full yaml files containing all categories.

Describe the solution you'd like

It would be nice to be able to load/store extensions from definition files (probably also yaml).

Additional context

One use case: "I have an extension of a standard terminology, but don't want to or can't include it in climate_categories. I want to convey my extended categorization together with a dataset using it."