ENH: support 'if-present' handling of relaxation coefficients

- new

  scalar relaxCoeff = 0;
  if (solution().relaxEquation(name, relaxCoeff))
  {
      relax(relaxCoeff);
  }

  // or
  scalar relaxCoeff = 0;
  solution().relaxEquation(name, relaxCoeff);

- old

  if (solution().relaxEquation(name))
  {
      relax(solution().equationRelaxationFactor(name));
  }

  // or
  scalar relaxCoeff = 0;
  if (solution().relaxEquation(name))
  {
      relaxCoeff = solution().equationRelaxationFactor(name);
  }