get the square root of the cofactor matrix attribute. Create the attribute if
        it has not yet been created

        Returns
        -------
        qhalf : pyemu.Matrix

def qhalf(self):
        """get the square root of the cofactor matrix attribute. Create the attribute if
        it has not yet been created

        Returns
        -------
        qhalf : pyemu.Matrix

        """
        if self.__qhalf != None:
            return self.__qhalf
        self.log("qhalf")
        self.__qhalf = self.obscov ** (-0.5)
        self.log("qhalf")
        return self.__qhalf