compute pixelwise gradients w.r.t. to classes
classify + localize, with notebook

- define nets for classification and gradient backprop on imagenet 1k
- given an image, classify then compute pixelwise gradients w.r.t.
  top activation
- demo in notebook
drop deprecated compute_gradients()
input + output pd data frames, adopt compute_feats_grads
support classification / classification + gradient localization use

- accept localize_def flag/param for the gradient backpropagation /
  localization net
- configure localization (when given)
- compute pixewise gradients w.r.t. top activation (when configured for
  localization)