differential: set a default initialisation

An initialization is required for all states variables.
Otherwise an exception is thrown. We modify this behavior
by setting a default initilialization value to 0 for variables
that are not explicitely initialized. (Closes #42)
differential: allow the update of state variables

- add an operator= for Var which updates the state value
- add a constructor that initializes the value
- add a const access operator to the state value (for observation)
differential: simplify the initialisation

Variables can now be initialised using the InitEventList as the container of
initialisedvalues. The vpz Ladybirdplantlouse is modified for testing
purpose (Closes #41).