Add option for Difference of Gaussians to be a single kernel

[krisharma]

Currently our Difference of Gaussians demo computes distinct Gaussian kernels over two instances of the same image and then subtracts the images to get the output. We want to add the option for users to be able to toggle between doing this or first creating two kernels, subtracting them and then sliding that singular kernel across the image and getting the same result.

We had some issue with this last semester so happy to discuss when it comes to implementation.