Trend that saturates on only one end

The logistic growth trend saturates at both lower and upper bounds. It would be useful to have a trend that saturates only on one end, e.g. that saturates at 0 but does not require specifying an upper bound.

This will require adding a new trend, adding to the linear and logistic growth trends that we already have. I think that the logistic loss function $log(1 + exp(k * (t - m)))$ should work very nicely.

If anyone is interested in making a PR for this we can discuss what is involved.