Implementation to allow custom namelist modification through configuration files.

conf/real-from-dwd-ana/simulation.yml

conf/real-from-ideal/simulation.yml

namelists/common/icon_master.namelist

namelists/real-from-dwd-ana/icon_atmosphere.namelist

namelists/real-from-ideal/icon_atmosphere_ideal.namelist

namelists/real-from-ideal/icon_atmosphere_real.namelist

namelists/real-from-ideal/icon_master.namelist

templates/real-from-dwd-ana/prepare_namelist.py

templates/real-from-ideal/prepare_ideal_namelist.py

templates/real-from-ideal/prepare_namelist.py