difference-equation: add a constructor field

The modeling plugin has now a new field that enable to add custom code
into the constructor method. From now on, this way, we can define
custom parameters.
(closes #50)
new packages: discrete-time and utils pkgs

Add a first version of new packages.
- vle.discrete-time.* which are intended to
model discrete time models and replace vle.extension.difference-equation
- vle.reader and vle.recursive are utils packages to read files and perorms
  embedded simulations
- tests packages are added for these new features.