Generate automatically unique_id

[luca-patrignani]

This PR aims to address issue #105 by generating automatically the unique_id for each agent and not letting the user assign it.
The proposed solution is very similar to the mesa's one which is adding static iterators (one for each model) to keep track of used ids.

Need confirmation

• When adding a new agent with a user-defined id the add method will throw a warning and ignore the id
• In the unit tests every assertion about unique_ids' content has been removed or changed