Normalize χ states before backward propagation #55
Description
It enhances the numerical stability of Krotov's method if the states |χ(T)⟩ that are the boundary condition for the backward propagation are normalized. The norm can the be included later in the pulse updates to compensate for this change.
It also allows to detect guess pulses at exact saddle points of the optimization landscape, which can happen with simple systems and some functionals (e.g., if the forward-propagation with the guess pulse yields a state that is exactly orhogonal to the target state).