More header cleanups in preparation for embedding.

We now have a self-consistent API for the model foundation classes:
installing the following header files in a separate directory

gmsh/
 GEdge.h -> ../Geo/GEdge.h
 GEdgeLoop.h -> ../Geo/GEdgeLoop.h
 GEntity.h -> ../Geo/GEntity.h
 GFace.h -> ../Geo/GFace.h
 GModel.h -> ../Geo/GModel.h
 GPoint.h -> ../Geo/GPoint.h
 GRegion.h -> ../Geo/GRegion.h
 GVertex.h -> ../Geo/GVertex.h
 GmshDefines.h -> ../Common/GmshDefines.h
 MEdge.h -> ../Geo/MEdge.h
 MElement.h -> ../Geo/MElement.h
 MFace.h -> ../Geo/MFace.h
 MVertex.h -> ../Geo/MVertex.h
 Pair.h -> ../Geo/Pair.h
 Range.h -> ../Geo/Range.h
 SBoundingBox3d.h -> ../Geo/SBoundingBox3d.h
 SPoint2.h -> ../Geo/SPoint2.h
 SPoint3.h -> ../Geo/SPoint3.h
 SVector3.h -> ../Geo/SVector3.h

and compiling a single "libGmsh.a" lib with "make embed" we can link
a test.cpp program like this

#include <gmsh/GModel.h>

int main(int argc, char **argv)
{
  if(argc > 1){
    GModel *m = new GModel();
    m->readMSH(argv[1]);
    m->writeMSH("out.msh");
  }
}

with

g++ test.cpp -lGmsh -lgsl -lgslcblas