Support for undefined gradients

Mixed3

(needed to express that gradient with respect to x is undefined
if x is allowed to be None)

added a new kind of bad grad, undefined grad

We don't want to do it at import time.

renamed FunctionGraph.extend to FunctionGraph.attach_feature

made extend call attach_feature

Tell how to get the full list.

No need to review in detail. This is sandbox stuff.

We where expecting only to execute it on 1 directory.

The problem was that we tested sqr(images2neibs(x)) with too big
values. This caused problem when we compute the numerical gradient.

This will fix some of the stochastic optimization order problem.

Will be used in a later commit.

fixed feature class versus instance issue