vis.contrast.naka_rushton_func

  vlt.fit.naka_rushton_func - Evaluate Naka Rushton function

   R=vlt.fit.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.

   See Peirce 2007