title: if (Mesh or Brep)'s face is plane,how to get its normal vector
body: <p>I want to click an entity's face, and occur its normal vector.It will help me to set a rotating axis.</p>
  
  <p>I follow model1_SelectionChanged in sample:EyeshotDemo,and i can know which face I click.But i don't know what can i do in next step.</p>