ENH/TST: Warn user if image looks like color (has 3 or 4 in its shape).

    In [2]: image = imread('becca_testimage.tif')

    In [3]: image.shape
    Out[3]: (250, 250, 4)

    In [4]: f = tp.locate(image, 21)
    UserWarning: I am interpreting the image as 3-dimensional. If it is actually a 2-dimensional color image, convert it to grayscale first.