'**SUMMARY**
This method converts the given RGB image to grayscale using the
Luminosity method.
**Parameters**
None
**RETURNS**
A GrayScale image with values according to the Luminosity method
**EXAMPLE**
>>> img = Image (\'lenna\')
>>> out = img.getLuminosity()
>>> out.show()
**NOTES**
Algorithm used: value =  0.21 R + 0.71 G + 0.07 B'

def getLuminosity(self):

    if ((self._colorSpace == ColorSpace.BGR) or (self._colorSpace == ColorSpace.UNKNOWN)):
        imgMat = np.array(self.getNumpyCv2(), dtype=np.int)
        retVal = np.array(np.average(imgMat, 2, (0.07, 0.71, 0.21)), dtype=np.uint8)
    else:
        logger.warnings('Input    a    RGB    image')
        return None
    return Image(retVal, cv2image=True)
