all: activate WITH_DOC & WITH_TEST compilation options
differential: update the extension

- contains Runge-Kutta 4 and QSS2 numeric methods (as parameters)
- can handle multiple variables into atomic models
- tests are added directly to the package
- only one example into vle.examples is kept : a model coupling a decision model,
a difference-equation model and a differential-equation model (smartgardener)