def fits_header_value(hdu, key):
    """ Take FITS hdu and a key, return value associated with key (or None if key absent).
        Adapted from package photrix, class FITS.header_value.
    :param hdu: astropy fits header+data unit object.
    :param key: FITS header key [string] or list of keys to try [list of strings]
    :return: value of FITS header entry, typically [float] if possible, else None. [string or None]
    """
    if isinstance(key, str):  # case of single key to try.
        return hdu.header.get(key, None)
    for k in key:             # case of list of keys to try.
        value = hdu.header.get(k, None)
        if value is not None:
            return value
    return None