change the logic in GenerateMesh():

- never reload the model file--we don't need this trick anymore!

- always only generate the mesh we are asked to generate (pressing
  "1D" should *always* mesh the edges; pressing "2D" should only mesh
  the surfaces, unless there is no 1D mesh). This makes the behavour
  much more predictable when changing mesh attributes interactively.

small fixes for mesh opti

cleanup

bidir db: phase 3 -- move all the old geo stuff into Geo/

bidi db: phase 2 -- continued

bidir db: phase 2 -- removed all (well, most of) the old data structures and
algorithms

force orientation of mesh to match the orientation of the geometry

added full support for incomplete second order elements (8-node quad,
20-node hexa, 15-node prism and 13-node pyramid):

- second order generation algorithm can now be told to generate either
  complete or incomplete elements

- MSH I/O routines can deal with all elements

- UNV I/O routines now support all the elements in the I-deas spec (I did
  my best to follow the spec for the ordering of the nodes, but I might have
  made some mistakes: PLEASE TEST IF YOU HAVE ACCESS TO A CODE OTHER THAN
  GMSH THAT CAN READ/WRITE UNV FILES)

- finished Nastran BDF implementation (read+write in all BDF formats:
  free field, small field & long field)
- improved UNV and Medit import
- added detection of bad vertex indices in all import routines
- fixed bug in 2nd order quad constructor

- done rewriting second order algo

- small optimizations here and there to speed up the vertex array creation
  (we should get rid of List_Add & co)

moved most OS-dependent functions in OS.cpp

characteristic lenghts, that would lead to HUGE meshes (this is one
of the most common causes for erroneous bug reports on the mailing list)

remove obsolete BGM structure

removing dead code

rewrote the smooth normal routines:

- the averaging is now done by clusters, which is much better for
  geometries that have sharp edges  (it's a little slower, but the
  visual results are much better)
- there is now only a single smooth normal container in the mesh
  (instead of one per surface)

- new button under the graphic window to temporarily disable mouse
  selection (speeds-up redrawing of very large models + permits to
  rotate/zoom-in a model in selection mode even when the whole screen
  is full of selectable entities--e.g. a surface mesh)

- new "lasso" selection mode (to select entities using the same kind
  of lasso as the lasso zoom: just Ctrl+click, then drag the mouse in
  selection mode; the shortcuts are the same as for the lasso zoom)

- it is now possible to unselect entities using the middle mouse button
  (only for the creation of physicals at the moment; not sure if it's
  useful in the other cases)

- new button in visibility browser to invert the current selection
  (very useful e.g. when multiple physical entities are associated
  with a given elementary entity, in order to "peel" away the model
  when adding new physicals; cf. philou)

- changed meaning of Escape shortcut (cancel lasso or toggle mouse
  selection) + restore standard fltk Escape handling for all dialog
  windows

- updated copyright string

- new mesh label mode (coordinates); all label types are now also
  available for mesh vertices

- added option in 'Print Option' dialog to disable printing of help
  strings

- added a comment string with the date when creating a new file

- new snapping grid for adding points in the GUI

make it possible to add second order vertices for a mesh read from
a file (without cad)

use it now with -partition N for N partitions
only works with tets and triangles
output should be added (useless for now)

Removed the last bits of the "Discrete Surface/Line" stuff.

Gmsh can now simply read a .msh file, and use it as the boundary mesh
of a 3D mesh. For an example on how to do this, cf.
demos/sphere-discrete.geo.

This complements nicely the STL remeshing stuff: Gmsh can now
for example very easily be used to extrude surface meshes that
come from other tools.