gmsh

- 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