design effect

[MamadouSDiallo]

Add code to compute the design effect for Taylor-based estimates.

The API is already designed to use a deff parameter. The remaining step is to add the associated code.

def estimate(
        self: TypeTaylorEst,
        y: Array,
        samp_weight: Array,
        x: Optional[Array] = None,
        stratum: Optional[Array] = None,
        psu: Optional[Array] = None,
        ssu: Optional[Array] = None,
        domain: Optional[Array] = None,
        fpc: Union[Dict, Array, Number] = 1.0,
        deff: bool = False,
        coef_variation: bool = False,
        remove_nan: bool = False,
    ) -> None