Enhancement of the OCC interface

Plane surfaces are now recognized and the
usual gmsh "cross" is drawn correctly

occ work

:i

start opencascade integration

more mesh edit work

new GModel::writeGEO (only implemented for gmshModel for now; goal is to be able
to implement this for other derived model types (like OCC) later, too)

force orientation of mesh to match the orientation of the geometry

fix missing SIGN in NR

added Shewchuk's robust geometrical predicates (since they can be duplicated in
Tetgen/triangle I've put them in the gmsh namespace)

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)

- Use correct vertex ordering for second order simplices in UNV format
(thanks to Romuald Conty <conty@phimeca.com>)

- MLine2 -> MLine3

fix crash when adding a geo entity while a mesh is displayed

changed name of second order elements (in prevision of addition of standard
new ones like 8-node quad, 20-node hex & co)

- 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)

second order

reduce dependencies by putting the selection buffer stuff in its own file