code: public static void diagMult(Matrix A, Vec b)
      {
          if(A.cols() != b.length())
              throw new ArithmeticException("Could not multiply, matrix dimensions must agree");
          for(int i = 0; i < A.rows(); i++)
              RowColumnOps.multRow(A, i, b);
      }
docs: Alters the matrix <i>A</i> so that it contains the result of <i>A</i>
  times a sparse matrix represented by only its diagonal values or
  <i>A = A*diag(<b>b</b>)</i>. This is equivalent to the code
  <code>
  A = A{@link #multiply(jsat.linear.Matrix) .multiply}
  ({@link #diag(jsat.linear.Vec) diag}(b))
  </code>
  @param A the square matrix to update
  @param b the diagonal value vector