Rates are not stored uniformly in a model

[SergeStinckwich]

If a model is built from ODEs equations, rates are stored in the model as KEExpression.
If a model is built from transitions, rates are stored as lexical closure (Smalltalk block).

We should choose one way or another.

[SergeStinckwich]

After looking at the code more carefully, lexical closure seems to be used only when you build transitions models from low-level code.