Plugin SpanningTree should work with curved mesh elements too :)

Hello Christophe,

I checked the behaviour of the Plugin(SpanningTree) with curved mesh elements: it actually works like a charm :). What happens is that each high-order vertex is associated to a forest without edges (which is thus never merged)... The only problem is that I allocate some memory for those useless forests (two int per high-order vertex). That being said, I have the feeling that this is not a big problem: what do you think?

This commit basically removes the comment in GMSH_SpanningTreePlugin::getHelp about curved mesh elements, and adds a useful toString method for 'debugging' the disjoint set union.

Bye, Nicolas.