Updated the interface and documentation of scan following James suggestions; I also implemented a map function using scan

We should fix the real reason. I make a patch as this was making the CrossentropySoftmaxArgMax1HotWithBias op not being inserted as a stabilization.

duplicated verify_grad

implemented yet

 Scan finally works with shared variables ! .. there are a few changes to the documentation of scan that need to be added that will follow soon

bugfix in Alloc.perform(). This bug happen if their was inplace modification to the output of Alloc. It was not reset.

Check how and when to use destroy_map to don't copy to it too often.