fix copy/paste: netgen->ann

simpler tetgen integration by Jozef Vesely

generalized the solver interface to allow permanent listening on a
socket for whoever wants to connect (gmsh -listen)

the polling should also be less CPU hungry in this new version

next step is to add new commands to the protocol so that solvers can
directly transmit data to merge (geometry, mesh, etc.) in addition to
messages and filenames

missing cast

fix time step printf format

missing cast

- new option to choose the mesh element labels (element number, elementary
  entity number, physical entity number or partition number)

doc trsf surf orientation option

fix memory leak + missing depend

ex of sphere meshed with hexas

Please don't load solver plugins by default: for the vast majority of users,
the structural solver options don't make any sense.

Just add Solver.Plugins=1 in your option file if you need that stuff.

finer control of time/timestep display

move msg

compile tetgen by default so we get feedback on machine-specific problems

try different ordering when swapping fails

fix display of normals for very "thin" surfaces with curved boundaries:
this new algorithm should be better

All modifications are commentarized because
this feature is not functionnal : the transfer from
Gmsh to Tetgen is OK, but the transfer from Tetgen to
Gmsh is not : all the mesh (points, lines,
surfaces, volumes) must be recreated in Gmsh...