Bug in gmsh api writer.

[luca-heltai]

I've added two tests that do the following things:

• create a hyper_cube triangulation
• write it using gmsh api
• read it back using gmsh api
• write it again using gmsh api
• copy the two meshes to the output file (they should be identical, up to maybe different node numberings in different versions of gmsh, but not for this simple case)

If only the default boundary id (0) is used, everything is ok (test gmsh_api_05). If I colorize the grid, then the gmsh api writer GridOut::write_msh(tria, string) fails to write nodes information (gmsh_api_06).

In the second case, I get as msh file the following:

$MeshFormat
4.1 0 8
$EndMeshFormat
$PhysicalNames
3
1 1 "BoundaryID:1"
1 2 "BoundaryID:2"
1 3 "BoundaryID:3"
$EndPhysicalNames
$Entities
0 3 1 0
2 1 0 0 1 1 0 1 1 0 
3 0 0 0 1 0 0 1 2 0 
4 0 1 0 1 1 0 1 3 0 
1 0 0 0 1 1 0 1 4 0 
$EndEntities
$Elements
4 4 1 4
1 2 1 1
2 2 4 
1 3 1 1
3 1 2 
1 4 1 1
4 3 4 
2 1 3 1
1 1 2 4 3 
$EndElements

Everything is fine, except there is no $Nodes section. I created the output of gmsh_api_06 by hand.

I have not worked on a fix yet, but when the fix is ready, the second test should pass.