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 (closed)
This should be backported to 2.5 and possibly also to 2.4. It is an easy, non-invasive backport.