Plans with species2nudge setting in icon-art-global-test

[mjaehn]

From #18.

cases/icon-art-global-test/config.py:

# Nudging (meteorological and tracers)
era5_global_nudging = False
species_global_nudging = False
species2nudge = []

Does species2nudge have any impact on the results or is this is only a feature to be implemented in the future? @jthanwer