def load_model(filename):
    """
    Return a model stored within a file.

    This routine is for specialized model descriptions not defined by script.

    If the filename does not contain a model of the appropriate type (e.g.,
    because the extension is incorrect), then return None.

    No need to load pickles or script models.  These will be attempted if
    load_model returns None.
    """
    return None
