four parameter logistic (first pulse only)

Per a suggestion of Dr. Modis, The first logistic pulse is now a four parameter logistic:


\displaystyle p(t) = \frac{\kappa}{ 1 + \exp\left( \frac{\ln(81)}{-\alpha} (t - \beta) \right)} + d

where d is a initial displacement value.
The default constraints on d are zero and zero, so the
default is to fit a three parameter logistic for the first pulse
(i.e. the guessing algorithm will not attempt to guess a displacement)