Add ChangeModel to dev

[raoulcollenteur]

Short Description

This PR add the ChangeModel to the stress models, implementing (on a basic level) the idea of Obergfjell et al (2019). It differs from that work, in that it can only take one stress (e.g., a river) as an input, not precipitation and evaporation. However, I still think it may be useful to some, and perhaps can be extended in the future to take more complicated functions into account.

Checklist before PR can be merged:

• closes issue #xxxx
• is documented
• PEP8 compliant code
• tests added / passed (through notebook)
• Example Notebook (for new features)
• API changes documented in Release Notes

[rubencalje]

Nica add-on. With this we can model the transition from one response function to another, or from one set of response-function-parameters to another set, which is great!

Would it in the future also be possible to model the transition from one stressmodel to another stressmodel? In that way, it would also work for recharge-models.

[raoulcollenteur]

Would it in the future also be possible to model the transition from one stressmodel to another stressmodel? In that way, it would also work for recharge-models.

Initially that is what I tried, but this proved very hard to implement due to how we set the parameters in that model (which depend on the response function and the recharge model). I think we can look at that in the future if someone needs that functionality though.

I'll add the API changes documented in Release Notes and merge the PR later today .