enable new Delaunay for plane surfaces

experimental: don't use it for now

- preliminary work to interface more parser functions with general
  GEntities

- we can now export simple GModels as.geo files (points, straight line
  segments, plane surfaces, ruled surfaces and volumes are treated
  correctly; all non-straight-line curves are exported as splines; all
  non-plane/ruled surfaces are not exported)

- tentative fix for the unv export problem reported on the mailing
  list (fprintf() buggy on some machines??)

- new IO (both read and write) for structured grids using the NASA
  Plot3D format (for Olivier)

general, but ProjectPointOnSurface for example would fail on plane
surfaces)

- temporary fix for creation of second order vertices on periodic
  curves and periodic surfaces

- speedup 2nd order vertex creation

- never create MFaceVertices if we don't know the parametric
  coordinates (this will screw up the 2nd order algo!): create
  MVertices instead

- fixed u,v coords of vertices after elliptic smoother in
  meshGFaceTransfinite

- added "-format msh1" and "-format msh2" shortcuts to quickly
  select the version of the msh format from the command line +
  added "med" and "cgns" formats

- moved gui guidelines in separate file

cleaned up InterpolateSurface/Curve (get the negative curve dircetly in InterpolateCurve)

reintroduced fix for transfinite mesh on 3-sided transfinite surfaces: the problem was still there

- removed ugly fix in transf mesh of tri surf
- use MFaceVertex params in boundary mesh if available
- cleanup intro chapter in docs

are more correct)

make sure the boundary vertices are ordered consistently with the
ordering of the transfinite corners. If not, just revert the list.

This is crucial for maintaining backward compatibility with the old
version.

slightly better fix than before

add extra arg to XYZtoUV so we can use it also for point that are not on
the surface

temporary (?) fix for transfinite on tri

reimplemented transfinite volume meshing algo (not complete yet)

don't build metis and ann by default anymore

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)