- All extrusion commands now return a list of 2 numbers (instead of 1):
  the first, as before, is the number of the "top" of the extruded region
  (i.e., a point for extrude point, a line for extrude line, ...), the
  second is the number of the "body" of the extruded region (i.e., a
  line for extrude point, a surface for extrude line, ...).

- "Extrude Surface" now always creates a new volume (automatically),
  EVEN WHEN THERE IS NO LAYERS SPECIFICATION. This makes it consistent
  with "Extrude Point" and "Extrude Line", which always create new
  curves and surfaces, respectively.

  Important Note: you will have to modify your old .geo files to avoid
  duplicate volume definitions if you use "Extrude Surface" without
  extruding the mesh (i.e., without the "Layers" command). These
  duplicate volumes would be harmless, but they would srew up your
  physical volume definitions later on...

  * Solution 1: use the new volumes (recommended). To do this, just
  remove your old extra volume definitions and let Gmsh create the
  extruded volumes for you. (To retrieve the volume number created by
  Gmsh, use "aa[] = Extrude Surface {...};;": the volume number is
  "aa[1]".)

  * Solution 2: keep the old volumes.

  a) clean way: retrieve the new volume number (aa[] = Extrude Surface
  {...};;) and delete the new volume with "Delete { Volume aa[1]; }"

  b) dirty (but handy) way: since, in order to create the new volumes
  with the less impact possible, Gmsh uses "low" numbers (actually,
  forcing "Geometry.OldNewreg=0") for the new volumes, just remove all
  "low number volumes". For example, if you have 4 "Extrude Surface" in
  your file, you can then just do "Delete{ Volume {1:4}; }"

Voila :-)