code: def getR(self, i=5, j=6):
          
          if self.refresh is True:
              self.getMatrix()
          return self.transM[i - 1, j - 1]
docs: return transport matrix element, indexed by i, j,
          be default, return dispersion value, i.e. getR(5,6) in [m]
  
          :param i: row index, with initial index of 1
          :param j: col indx, with initial index of 1
          :return: transport matrix element