message: implement getrow for union dataset
diff: @@ -236,7 +236,12 @@ struct UnionDataset::Itl
  // DEPRECATED
  virtual MatrixNamedRow getRow(const RowPath & rowPath) const
  {
  - throw MLDB::Exception("Unimplemented %s : %d", __FILE__, __LINE__);
  + int idx = getIdxFromRowPath(rowPath);
  + if (idx == -1) {
  + return MatrixNamedRow();
  + }
  + return datasets[idx]->getMatrixView()->getRow(
  + Path(rowPath.begin() + 1, rowPath.end()));
  }
  virtual bool knownColumn(const Path & column) const
  