Draft: Script for CMIP7 emissions

Sorry for the large merge request...

@b309219 the tests are still work-in-progress, but you can already have a look at the code if you have time.

@b309220 @b309253 @b302019 @b309098 your comments are also welcome.

A first subset (2000-2023) of the processed emissions can be found at /scratch/b/b309057/CMIP7: please have a look and let me know if there is anything to change (metadata, attributes, filenames, etc.)