Improve to_initialize(mapping, graph)

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The method is way too complicated and its implementation is very crude. We could do what it does with much fewer lines of code, and also more elegantly. For example, we should recursively search for variables that are inputs but computed at another scale, and it may even be possible to get the scale at which a variable is computed without giving it explicitly (just by searching for it as an output).

In this case we could have a method for MultiScaleModel with just the model, and without the mapping. Then our function would search for the variable at other scales, and if it finds several occurrences, it would guide the user on how to use the two-argument method of MultiScaleModel for the different possibilities.

[VEZY]

Closed in f7de08b