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 support for co2 concentration and sending data from multiple variable to one oasis var (e.g. CO2-em-anthro and CO2-em-AIR-anthro)