Improve nonlinear update output

[landinjm]

The current output for each nonlinear solve is not very descriptive.

if (currentIncrement % userInputs.skip_print_steps == 0)
  {
    pcout << "Relative difference between nonlinear "
             "iterations: "
          << diff << " " << nonlinear_it_index << " " << currentIncrement
          << "\n";
  }