- in continuation of #2565 (rotationCentre for surface output formats)
  it is helpful to also support READ_IF_PRESENT behaviour for the
  'origin' keyword.

  This can be safely used wherever the coordinate system definition
  is embedded within a sub-dictionary scope.

  Eg,
      dict1
      {
          coordinateSystem
          {
              origin (0 0 0);  // now optional here
              rotation ...;
          }
      }

   but remains mandatory if constructed without a sub-dict:

      dict2
      {
          origin (0 0 0);   // still mandatory
          e1  (1 0 0);
          e3  (0 0 1);
      }

   With this change, the "transform" sub-dictionary can written
   more naturally:

       formatOptions
       {
           vtk
           {
               scale 1000;  // m -> mm
               transform
               {
                   rotationCentre  (1 0 0);
                   rotation axisAngle;
                   axis    (0 0 1);
             ...