Select channel in setLineScan/getLineScan

Added a channel select option in Image.getLineScan/Image.getLineScan functions. 

Now the user can work any color space and select any valid channel from an image and pull out a linescan. 

Example:

    img = Image("lenna")
    ls = img.getLineScan(x=200,channel=0)
    for i in range(len(ls)):
        ls[i] = 0
    img = img.replaceLineScan(ls)
    img.show()

this code will select the Red channel and set it to zero in the image.