[REVIEW] Add "real" multi-component leaf/plant model example

[ashiklom]

The current multi-model example in Model-Switching (https://vezy.github.io/PlantSimEngine.jl/stable/model_switching/) is a bit esoteric. Unless you're good at software engineering already (which is not the intended audience of this package), that makes it a bit tricky to see the value of the approach.

I suggest including a more real example -- even if it's a really basic one.

openjournals/joss-reviews#5371

[VEZY]

OK these last commits should do it. Let me know if you have any other request or if that's not clear enough.