Apply vertex reordering to boundary segment vertices

UG wants all boundary vertices to be listed before the other ones.
Therefore, the UGGrid GridFactory reorders the vertices before
they get inserted into the actual UGGrid object.

Unfortunately, vertices of explicitly given boundary segments
used the wrong numbering.  Therefore, constructing grids with
explicitly given boundary segments would fail, unless the
vertices happened to be in the correct order.

This fixes staging/dune-uggrid#18

dune/grid/uggrid/uggridfactory.cc

@@ -327,7 +327,7 @@ createGrid()
     // We copy four vertices here even if the segment is a triangle -- it doesn't matter
     int vertices_c_style[dimworld*2-2];
     for (int j=0; j<dimworld*2-2; j++)
-      vertices_c_style[j] = boundarySegmentVertices_[i][j];
+      vertices_c_style[j] = isBoundaryNode[boundarySegmentVertices_[i][j]];
 
     if (grid_->boundarySegments_[i]) {