docs: public static double diffNormP1(DMatrixD1 a , DMatrixD1 b )
      {
          if( a.numRows != b.numRows || a.numCols != b.numCols ) {
              throw new IllegalArgumentException("Both matrices must have the same shape.");
          }
  
          final int size = a.getNumElements();
  
          double total=0;
          for( int i = 0; i < size; i++ ) {
              total += Math.abs(b.get(i) - a.get(i));
          }
          return total;
      }