/// - the slope can be adjusted using C > 0, smaller C giving a steeper sigmoid
/// - the slope of the sigmoid when t = x0 is P/(Q*C)
/// - sigmoid is increasing with t when P > 0 and Q > 0
/// - the slope can be adjusted using C > 0, smaller C giving a steeper sigmoid
/// - the slope of the sigmoid when t = x0 is P/(Q*C)
/// - sigmoid is increasing with t when P > 0 and Q > 0