Clean-up & small syntax change for homology commands.

Added homology computation tutorial as tutorial/t10.geo.

- display all control points in GUI for gmsh curves
- uniformize Fl_Value::step() values in GUI

- print num vert+elm after each generation
- reork tutorial a bit

pp

Layers{N,1} can now be written simply as Layers{N}

physical entities can now be defined using a string instead of a number (and
there is a new field $PhysicalNames in the .msh format to keep track of these
strings)

- re-add names in visibility broswer (will change in the future)
- shorten tutorials

better tutorial on transfinite grids

remove more attractor crap

don't use the "manual" numbering the extrude demo: it creates bad habits
for new users

fixed ugly extrusion syntax (finally!)

extrusions are now specified in the same way as all other transformations
(the old syntax is still available, but is deprecated)

refactored Opengl_Window.handle()

should behave exactly like the old version, but now it is at least (somewhat)
readable and ready to be generalized for more complicated projection matrices

a little more work on the axes stuff: enable global axes for the geometry and
the mesh, too. We can finally get rid of the ugly old "moving axes" bit.

(+ reverted changes in unproject: need to be more careful since unproject
can be called outside Draw())

add special token "Today" to return the current date

by convention, for 2d strings, assume that a coord value > 99999 means the
center of the window

add title

simpler Sprintf

pretty print

document the optional second list in Layers

Removed the old test on ZonLayer: we actually WANT to use the automatic
volume if we set the layer number to 0. This is really pretty nice: we
can now get either automatic or manual numebring in all the extrusion
commands.

- 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 :-)

refactor the netgen code in a dedicated class

Merged Netgen 4.3.1: a Delaunay/Frontal 2D/3D mesh generator.

Thanks a lot to Nicolas Tardieu for this!

Small changes inspired by Wednesday's discussions:

- Mac only: Gmsh now publishes its "file types"... That means that
  geo/msh/pos files now have nice icons, and that you can double-click
  on a geo/msh/pos file in the Finder to open it :-)

- removed support for fltk 1.0 (wasn't compiling anymore anyway)

- doubled the timout in GmshServer (2->4s): this seems to help on
  Windows (thanks to jkools@veeco.com)

- added warnings in the docs about the Attractor feature

- increased WB by 1 pixel in the GUI

- replaced more Msg(FATAL) with Msg(ERROR) where it makes sense