ENH: Convert Criterion into Operators

[pchanial]

... to benefit from all the Operator's goodies.

[nbarbey]

This require :

• Implementing nonlinear operators : Power, Sqrt, Sum (reduction ?)
• or directly Norm2, Normp and Huber nonlinear operators
• Adding a diff method to those operators which would return an Operator instance corresponding to the differentiation of this operator

Comments:

• Possibly the diff operator has itself a diff method. In this case a diff2 or hessian shortcut would come handy.
• To ease the implementation of the diff methods AdditionOperator, CompositionOperator, ... should propagate the diff methods in accordance to differentiation rules (chain rules, ...).
• So it seems easier to implement the "automatic differentiation" issue before this one.
• Once this is done, no other modification of the iterative algorithm should be required as the only expectation is an object with a call and a diff method.