Repartition interface with PTScotch

The graph partitioning interface added in !3005 (merged) assumes that the initial graph is on rank 0. This is useful to distribute a grid read on rank 0 e.g. with the gmsh reader. However, for dynamic load balancing, we need a repartition interface that works if the graph is distributed. PTScotch provides dgraph (distributed graph) for this purpose. Distributed graphs have a more complex interface but include also the current case where the entire graph is on rank 0 (all other ranks then have 0 graph nodes).