From Olivier

- Paint the grid levels on the triangles along the Z axis
- Option P  : draw a marker at each vertex
- Option P0 : draw a circle at each vertex. Each circle background is
  white
- Mods in Fit (needed by TGraph2DErrors)
- New virtual methods to get the errors (needed by TGraph2DErrors)


git-svn-id: http://root.cern.ch/svn/root/trunk@7659 27541ba8-7e3a-0410-8455-c3a389f83636