ArgumentParser::add_settings_file to add positional argument for settings file.
special case if TERRA_SETTINGS_FILE is already defined, using TERRA_SETTINGS_FILE as the default value. Users may still enter an alternative settings file at the command line if desired.
users may also override add_argument kwargs, for example changing the help statement.
ArgumentParser::add_settings_fileto add positional argument for settings file.TERRA_SETTINGS_FILEis already defined, usingTERRA_SETTINGS_FILEas the default value. Users may still enter an alternative settings file at the command line if desired.add_argumentkwargs, for example changing the help statement.