support for asymmetric limits

[antoine-sac]

In cases where the actionner is asymmetric, the limit should also be asymmetric.

The most common case will be 0 <= output <= limit.

For example, consider a non-reversible heater maintaining room temperature, which just so happens to be my main use case :)

Let's say a 1500W heater is supposed to maintain a room at 19ºC, with a current temperature of 22ºC. The poor integral term will try its best to lower the temperature until it saturates at -1500W, which makes little sense.

But we might as well support arbitrary limits.