octree + CutGrid,CutParametric,Probe for model post views

added nodal interpolation functions & related stuff

remove BGMReset

re-fix Field.h API dependency

fix shift/conflict reduce in parser introduced by Field stuff

change new post API to allow multiple meshes in a single view (different
meshes in different steps)

- temp fix I/O to allow skipping $xx fields (this will be common
  with new post-pro)
- prune MElement (move mesh-only stuff in mesh algos)

selective loading of steps)

- new GmshInitialize(argc, argv) and GmshFinalize() for library version,
  now also used throughout

- GModel::mesh(dimension)

We now have a simple API to do basic stuff. We'll interface more as we go (optimize, adapt, etc.).

#include <stdio.h>
#include <gmsh/Gmsh.h>
#include <gmsh/GModel.h>
#include <gmsh/MElement.h>

int main(int argc, char **argv)
{
  GmshInitialize(argc, argv);
  GModel *m = new GModel();
  m->readGEO("./tutorial/t5.geo");
  m->mesh(3);
  for(GModel::riter it = m->firstRegion(); it != m->lastRegion(); ++it){
    printf("volume %d contains %d elements:\n", (*it)->tag(), (*it)->getNumMeshElements());
    for(unsigned int i = 0; i < (*it)->getNumMeshElements(); i++)
      printf(" %d", (*it)->getMeshElement(i)->getNum());
    printf("\n");
  }
  m->writeMSH("test.msh");
  m->writeUNV("test.unv");
  GmshFinalize();
}

- notion of partitions and interpolationScheme
- move string utils in separate file

data asap
* finer estimate of mem usage