Creates a face with the vertices 'eyeVertex.point', 'horizonEdge.tail' and 'horizonEdge.head' in CCW order
function(eyeVertex, horizonEdge) {
      // all the half edges are created in ccw order thus the face is always pointing outside the hull

      var face = Face.create(eyeVertex, horizonEdge.tail(), horizonEdge.head());

      this.faces.push(face);

      // join face.getEdge( - 1 ) with the horizon's opposite edge face.getEdge( - 1 ) = face.getEdge( 2 )

      face.getEdge(-1).setTwin(horizonEdge.twin);

      return face.getEdge(0); // the half edge whose vertex is the eyeVertex
    }