reccurence-relation: first proposal

- DEVS state and transition proposed
- first version of implementation
- generic model proposal
recurrence-relation: new DEVS state proposal

- no initialisation process
- no propagation of perturbation
recurrence-relation: new implementation

- implementation of the DEVS model
- remove temporarly generic model and tests
recurrence-relation: add a VLE pkg for tests

- vle.extension.recurrence-relation_test provides tests based on
  difference-equation extension.
- first model (equation1.vpz) is partially implemented.