Test writeneighbors method

[wo80]

The writeneighbors method in triangle.c (mis)uses the first subseg pointer of the triangle data structure for triangle numbering. This was not an issue for the original Triangle code, since the method was called last.
With the new Triangle api, the mesh data is reusable and so the original subseg pointers have to be restored. This needs some testing.

Additionally, the file_writeneighbors method in triangle_io.c needs the same updates!