'return the average color within the circle
**SUMMARY**
Return a numpy array of the average color of the area covered by each Feature.
**RETURNS**
Returns an array of RGB triplets the correspond to the mean color of the feature.
**EXAMPLE**
>>> img = Image("lenna")
>>> kp = img.findKeypoints()
>>> c = kp.meanColor()'

def meanColor(self):

    if (self._avgColor is None):
        TL = self.topLeftCorner()
        raw = self.image.crop(TL[0], TL[0], self.width(), self.height())
        mask = Image((self.width(), self.height()))
        mask.dl().polygon(self._minRect, color=Color.WHITE, filled=TRUE)
        mask = mask.applyLayers()
        retVal = cv.Avg(raw.getBitmap(), mask._getGrayscaleBitmap())
        self._avgColor = retVal
    else:
        retVal = self._avgColor
    return retVal
