[scikit-image] n-dimensional Sobel filter

Hi everyone, To my horror as Guardian of the Dimensions, I discovered yesterday that we have *no* n-dimensional edge filters in scikit-image! I was preparing to give a workshop and update the 3D image processing example when I found this. I’ll submit a PR soon but the outcome was clean enough that I wanted to share with you ahead of time my n-dimensional Sobel filter here: https://nbviewer.jupyter.org/github/jni/skimage-tutorials/blob/monash-df2-co... <https://nbviewer.jupyter.org/github/jni/skimage-tutorials/blob/monash-df2-co...> (search for "def sobel”). I want to allow an axis= kwarg and then replace our 2D implementations with calls to this one. =D Many of our other filters are similarly generalisable. Juan.