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.