docs: public static void concatRows(DMatrixRMaj a , DMatrixRMaj b , DMatrixRMaj output ) {
          int rows = a.numRows + b.numRows;
          int cols = Math.max(a.numCols , b.numCols);
  
          output.reshape(rows,cols);
          output.zero();
  
          insert(a,output,0,0);
          insert(b,output,a.numRows,0);
      }