Improve how computations are made over MTGs

[VEZY]

See #38.

The features that we need to have:

1. (already there) associate a set of models to a symbol using a ModelList
2. get access to variables from other scales if we need to
3. make this access fully transparent without the need to understand how MultiScaleTreeGraph works or where the values of a variable are. I.e., access the variable using status.A, not e.g. node[:models].status[timestep][:carbon_demand].

To do:

• 1. We can now provide a mapping between organs and models. This mapping is then used to compute a multiscale dependency graph.
• For 2. and 3., we need to cache the tree traversal into an object, but not the nodes, directly the ModelLists, or even better, the Status. Problems:
  • These statuses are dynamic, as new organs can appear on the plant and older organs can die off. So each time the MTG is updated (adding or removing an organ), we must update this structure accordingly (see fa528e1).
  • We usually expect a singleton when doing status.var, but if we ask variables from several organs at a time (e.g. assimilation from all leaves in the plant), we will get a vector. As stated before, the number of organs can change over time, so we should always provide a vector of values because a singleton can become a vector dynamically, which is a nightmare to handle. Another way to manage all this is to pass the values via the extra field, but in this case, we would lose the ability to traverse the mtg from inside a model. This is not acceptable as we use it for adding/removing organs. OK so the solution is to use vectors inside the Status. Those vectors should be typed (we know the types, so it's OK, and ideally, they shouldn't copy the value but be a Vector of Ref instead. => This is done, we now pass the variables as RefVectors to other scales if the user map the variables as e.g. :var => ["Leaf"], and as a singleton if the mapping is e.g. :var => "Leaf". So in the end, it is the user that tells us if only one organ is expected during the simulation.
• Include a test on models when the model has theIsObjectIndependent()trait. In this case, it should check that variables from other scales are given as input to the model.

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[VEZY]

OK so what I did is to implement RefVector, which is a structure that holds a vector of references to values in a vector of Status. This way we don't make copies, and if we update the value from this vector, it will also update the value in the status.