Add a way to not use an input variable to build the dependency graph.

[VEZY]

We need a way to tell PlantSimEngine to not use an input variable to build the dependency graph.
This happens when e.g. we have a model that computes the light interception using the plant LAI at the beginning of step i, but the LAI is computed at the end of step i because it requires all kinds of computations first (e.g. leaf biomass increment). So because the interception model requires LAI as input, we naïvely put it as a child node of the LAI computation model, which makes a cyclic dependency graph (the last node is linked to the first node).

The solution is to be able to tell PSE to not use a particular variable for building the dependency tree, so the whole dependency graph does not become cyclic. We can do that by flagging the variable in the status using a structure, e.g.:

Status(LAI = PreviousTimeStep(1.0),)

[VEZY]

We can't use the proposition above because the Status is given for all models of the same scale (or for the whole ModelList). What we want is to flag this variable for a particular model instead. So we have no choice but use another way that flags the variable with PreviousTimeStep for a model.

We could ask users to use the MultiScaleModel structure to flag the variable even though the variable is not multiscale per se, e.g.:

MultiScaleModel(
    model=ToyXModel(),
    mapping=[PreviousTimeStep(:soil_water_content)],
)

And it would work with real multiscale variables too:

MultiScaleModel(
    model=ToyXModel(),
    mapping=[PreviousTimeStep(:soil_water_content) => "Soil"],
)

[VEZY]

Users can still provide a custom initialization value in the Status.