A simple container holding the non-final variables used in the iterations. Making the current state of the solver visible from the outside is necessary, because during the iterations, {@code x} does notexactly hold the current estimate of the solution. Indeed, {@code x} needs in general to be moved from the LQ point to the CG point.Besides, additional upudates must be carried out in case {@code goodb} isset to {@code true}.
In all subsequent comments, the description of the state variables refer to their value after a call to {@link #update()}. In these comments, k is the current number of evaluations of matrix-vector products.