naka_rushton_func

  NAKA_RUSHTON_FUNC - Evaluate Naka Rushton function

   R=NAKA_RUSHTON_FUNC (C, C50, N, S)

   Returns Naka Rushton function:  C^N/(C^(N*S)+c50^(N*S))

   If S is not specified, S is assumed to be 1.
   If C is negative, the result is negative.