code: public function cofactor()
      {
          $c = new self($this->size->rows, $this->size->cols);
  
          for ($m = 0; $m < $this->size->rows; $m++) {
              $arr_row = array();
  
              for ($n = 0; $n < $this->size->cols; $n++) {
                  if ($this->size->cols == 2) {
                      $arr_row[] = pow(-1, $m + $n) * $this->subMatrix($m, $n)->get(0,0);
                  } else {
                      $arr_row[] = pow(-1, $m + $n) * $this->subMatrix($m, $n)->det();
                  }
              }
  
              $c->addRow($arr_row);
          }
  
          return $c;
      }
docs: Gets the cofactor matrix
  
  @access public
  @return Matrix