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)
forrester: add forrester modeling extension

- vle.forrester is the extension for using Forrester formalism
- forrester relies on muParser which is also added
exs: fix missing RandomGenerator constructor

Into counter examples, the constructor was missing.
de: remove unecessary glue directory

Glue is now into anther vle package and is removed from
the difference-equation extension