recipe for operator splitting

This recipe has two parts. The first explains how to load data from a vector belonging to a different system. The second shows how to calculate corrections using DifferenceSquaredAdapter.

Recipe is accompanied by a model solving one phase unsaturated flow in porous medium (Richards eq), and two component contaminant transport using cell-centered finite volume scheme. Water flow is solved separately, contaminant transport uses iterative operator splitting to solve advection and diffusion separately (no dispersion).