'**SUMMARY**
This method converts the given RGB image to grayscale using the
Lightness method.
**Parameters**
None
**RETURNS**
A GrayScale image with values according to the Lightness method
**EXAMPLE**
>>> img = Image (\'lenna\')
>>> out = img.getLightness()
>>> out.show()
**NOTES**
Algorithm used: value = (MAX(R,G,B) + MIN(R,G,B))/2'

def getLightness(self):

    if ((self._colorSpace == ColorSpace.BGR) or (self._colorSpace == ColorSpace.UNKNOWN)):
        imgMat = np.array(self.getNumpyCv2(), dtype=np.int)
        retVal = np.array(((np.max(imgMat, 2) + np.min(imgMat, 2)) / 2), dtype=np.uint8)
    else:
        logger.warnings('Input    a    RGB    image')
        return None
    return Image(retVal, cv2image=True)
