Use norm(rhs) in CG instead of norm(r0)
Use norm(rhs) in CG instead of norm(r0)

The use of initial residual norm for scaling of the error resulted in
incosistent results when solver was called with good approximation. Use
of the RHS norm for scaling of the error allows calling the solver
several times for the same solution vector.

ddemidov/amgcl,dondieselkopf/amgcl,ddemidov/amgcl,dondieselkopf/amgcl,huahbo/amgcl,ddemidov/amgcl,ddemidov/amgcl,huahbo/amgcl,dondieselkopf/amgcl,huahbo/amgcl