- background: for some application it can be useful to have fully
  sorted points. i.e., sorted by x, followed by y, followed by z.

  The default VectorSpace 'operator<' compares *all*
  components. This is seen by the following comparisons

  1.  a = (-2.2 -3.3 -4.4)
      b = (-1.1 -2.2 3.3)

      (a < b) : True
      Each 'a' component is less than each 'b' component

  2.  a = (-2.2 -3.3 -4.4)
      b = (-2.2 3.3 4.4)

      (a < b) : False
      The a.x() is not less than b.x()

  The static definitions 'less_xyz', 'less_yzx', 'less_zxy'
  instead use comparison of the next components as tie breakers
  (like a lexicographic sort).
  - same type of definition that Pair and Tuple2 use.

      a = (-2.2 -3.3 -4.4)
      b = (-2.2 3.3 4.4)

      vector::less_xyz(a, b) : True
      The a.x() == b.x(), but a.y() < b.y()

   They can be used directly as comparators:

      pointField points = ...;

      std::sort(points.begin(), points.end(), vector::less_zxy...