code: static Frame toFrame(Matrix m, Key key) {
      final int log_rows_per_chunk = Math.max(1, FileVec.DFLT_LOG2_CHUNK_SIZE - (int) Math.floor(Math.log(m.cols()) / Math.log(2.)));
      Vec v[] = new Vec[m.cols()];
      for (int i = 0; i < m.cols(); ++i) {
        v[i] = makeCon(0, m.rows(), log_rows_per_chunk);
      }
      Frame f = new FrameFiller(m).doAll(new Frame(key, v, true))._fr;
      DKV.put(key, f);
      return f;
    }
docs: Helper to convert a Matrix into a Frame
  @param m Matrix
  @param key Key for output Frame
  @return Reference to Frame (which is also in DKV)