Better support for partial inference
I have added the partial-const branch for tweaks that improve NIFTy's support for partial inference.
The current idea is that one first builds the full Hamiltonian (as before), and then obtains a simplified version for partially constant inputs by calling its method simplify_for_constant_input() with the constant input components.
All that needs to be done (I hope) is to specialize this method for all composed operators, i.e. those that call other operators internally.
@kjako, @reimar, @parras, @pfrank: does the principal idea look sound to you?