message: GetGroupId: added sanity check.
diff: @@ -337,12 +337,22 @@ bool operator<(const ParNCMesh::CommGroup &lhs, const ParNCMesh::CommGroup &rhs)
  return lhs.size() < rhs.size();
  }
  +static bool group_sorted(const ParNCMesh::CommGroup &group)
  +{
  + for (unsigned i = 1; i < group.size(); i++)
  + {
  + if (group[i] <= group[i-1]) { return false; }
  + }
  + return true;
  +}
  +
  ParNCMesh::GroupId ParNCMesh::GetGroupId(const CommGroup &group)
  {
  if (group.size() == 1 && group[0] == MyRank)
  {
  return 0;
  }
  + MFEM_ASSERT(group_sorted(group), "invalid group");
  GroupId &id = group_id[group];
  if (!id)
  {
  