Fix reading a Gmsh file into a parallel UGGrid
Fixes #553 (closed).
Depending on the Dune version, the boundary markers are present on all processes (<= 2.6) or on the root process only (>= 2.7). Try to handle this in a flexible way in the corresponding data handle: Determine if the minimum size over all processes of the boundary markers vector is zero. If yes, assume that the root process contains all markers and broadcast them.
Currently, it isn't possible to refine a parallel UGGrid that has been read by a GmshReader, see dune-grid issue 83. Therefore, don't refine in the corresponding test.