Preconditioners for linear problems. To speed up convergence of the iterative solvers, preconditioners are often necessary. Use the method {@link smt.iter.IterativeSolver#setPreconditioner(smt.iter.prec.Preconditioner) IterativeSolver.setPreconditioner} to set the preconditioner. The default preconditioner is {@link smt.iter.prec.IdentityPreconditioner IdentityPreconditioner}, which does nothing.