title: How to divide points into two sets - upper and lower part of profile
body: <p>I have lists of points in 3d (x,y,z)
  For each list I want to divide that list into to two lists, one containing points from upper part of profile and second the lower half, just like that:</p>
  
  <p><img src="https://i.stack.imgur.com/WsaIs.jpg" alt="enter image description here"></p>
  
  <p>My question here is how to determine which point should go to upper-part and which one should go to lower part just from having those points with their coordinates (x,y,z).</p>