Multiple soliton solutions for the new <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="mml45" display="inline" overflow="scroll" altimg="si45.gif"><mml:mrow><mml:mo>(</mml:mo><mml:mn>2</mml:mn><mml:mo>+</mml:mo><mml:mn>1</mml:mn><mml:mo>)</mml:mo></mml:mrow></mml:math>-dimensional Korteweg–de Vries equation by multiple exp-function method