The ASCII file format of Vgrid is defined as follows:
- First comes global size and version information
  #vertices #elements #partitions   version-string

- Next comes vertex information (#vertices lines):
  gid pid sid x y z cid fx fy fz
  Here
  - gid  is the global vertex id in [0,#vertices-1]
  - pid  is the partition id  in [0,#partitions-1]
  - sid  source id for NeuroFEM (ignored in HeadFEM)
  - x,y,z  are vertex coordinates
  - cid  is a constraint id (for HeadFEM) 0 <= cid < 8,
    where the meaning is
    - 0 : no constraint
    - 1 ... 6: some of x,y,z fixed ... 
    - 7 : x,y,z fixed
  - fx, fy, fz  is the initial force (or prescribed displacement) on the vertex.
    In general zero.
- Finally there is the element information
  gid pid elemtype v1 ... vn(elemtype)
  - gid is the global element id
  - pid is the element's partition
  - elementype gives the type of the element and material number:
    0..99     is tetrahedron, the number gives the material id of the element
    100..199  is a cube, number-100 gives the material id of the element.
 - v1 ... vn are the global vertex ids of the element (4 or 8, depending on elemtype)

