Return data path to `filename`, checking locally first then in the config file.

def datapath4file(filename, ext:str='.tgz', archive=True):
    "Return data path to `filename`, checking locally first then in the config file."
    local_path = URLs.LOCAL_PATH/'data'/filename
    if local_path.exists() or local_path.with_suffix(ext).exists(): return local_path
    elif archive: return Config.data_archive_path() / filename
    else: return Config.data_path() / filename