add support for GFAS fire flux and proper management of grids, depending on which dataset is used (CEDS or CARBON_TRACKER)

several changes for co2box model in tests/ : use global (not hemispheric) co2 box model and deactivate co2 emissions in IFS_mock.py ; updates to convert-ece4pyreader-grids.sh

add DataCouplerCO2Box.py and associated namelist and namcouple, identical in functionality to DataCouplerAMIP.py