Make SSP2-NPi2025 the new default

[dklein-pik]

Purpose of this PR

Change default values im main.gms to values used for SSP2-NPi2025 in config/scenario_config.csv

[dklein-pik]

Hi @LaviniaBaumstark , this is my draft. However, changing the defaults in main.gms means changing the defaults in all scenarios, where the respective setting is empty (= uses the default from main.gms). Thus, when changing the defaults in main.gms we must decide if either all of these scenarios should use the new defaults, or if they should use the old defaults. The latter would require manually setting them to the old defaults in all scenario_config*.csv files.