et to a default based on the data type.

        Examples
        --------
        >>> for dt in [np.int32, np.int64, np.float64, np.complex128]:
        ...     np.ma.array([0, 1], dtype=dt).get_fill_value()
        ...
        999999
        999999
        1e+20
        (1e+20+0j)

        >>> x = np.ma.array([0, 1.], fill_value=-np.inf)
        >>> x.fill_value
        -inf
        >>> x.fill_value = np.pi
        >>> x.fill_value
        3.1415926535897931 # may vary

        Reset to default:

        >>> x.fill_value = None
        >>> x.fill_value
        1e+20

        Nr