A sigmoidal selectivity function with 50% selectivity at
weight sigmoidal_weight and width sigmoidal_sigma.
$$S(w) = \left(1 + \left(\frac{w}{\text{sigmoidal_weight}}\right)^{-\text{sigmoidal_sigma}}\right)^{-1}$$
See also
gear_params() for setting the selectivity parameters.
Other selectivity functions:
double_sigmoid_length(),
knife_edge(),
sigmoid_length()
