docs: public static void convert(DMatrix input , DMatrix output ) {
          if( output instanceof ReshapeMatrix ) {
              ((ReshapeMatrix)output).reshape(input.getNumRows(),input.getNumCols());
          } else {
              if (input.getNumRows() != output.getNumRows())
                  throw new IllegalArgumentException("Number of rows do not match");
              if (input.getNumCols() != output.getNumCols())
                  throw new IllegalArgumentException("Number of columns do not match");
          }
  
          for( int i = 0; i < input.getNumRows(); i++  ) {
              for( int j = 0; j < input.getNumCols(); j++ ) {
                  output.unsafe_set(i,j,input.unsafe_get(i,j));
              }
          }
      }