#555

Request to merge #555 into the master branch.
Thanks! That actually helped. Now, 'up' refers to the upside as seen in the image, not the top row of the numpy array. I will edit the docstring to say this explicitly. 

`Convolve` handles only instances of GSObject or ChromaticObject classes and cannot handle Image instances. So, I guess we should just stick with the way we perform convolution.