Return all annual maximum flow (`*.am`) files in cache folder and sub folders.

    :return: List of file paths
    :rtype: list

def amax_files():
    """
    Return all annual maximum flow (`*.am`) files in cache folder and sub folders.

    :return: List of file paths
    :rtype: list
    """
    return [os.path.join(dp, f) for dp, dn, filenames in os.walk(CACHE_FOLDER)
            for f in filenames if os.path.splitext(f)[1].lower() == '.am']