code: 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;
      }
docs: <p>
  Computes the p=1 p-norm of the difference between the two Matrices:<br>
  <br>
  &sum;<sub>i=1:m</sub> &sum;<sub>j=1:n</sub> | a<sub>ij</sub> - b<sub>ij</sub>| <br>
  <br>
  where |x| is the absolute value of x.
  </p>
  <p>
  This is often used as a cost function.
  </p>
  
  @param a m by n matrix. Not modified.
  @param b m by n matrix. Not modified.
  
  @return The p=1 p-norm of the difference matrix.