BUG: newton_krylov() was ignoring norm_tol argument, closes #4259

Yes, I thought that as well.  However, `nonlin_solve` uses `tol_norm=None` as a default.  When this gets passed to `TerminationCondition.check` you end up with a problem because `None` is not callable.  So either `nonlin_solve` has to use `tol_norm=maxnorm` as a default, or we have the changes above.