Add hierarchy between Service and Characteristic

Now, Service and Characteristic are not linked together by any relation. So, we need to provide the `handle`, `group_end_handle`, `value_handle` and `config_handle` manually.

However, it could be interesting to add a method to add a characteristic to a service (meaning that it belongs to this service). This way, most of the handles will be automatically calculated. Perhaps, letting an option to let the user choose them manually would be interesting too.

**Proposal**:
- add an `add_characteristic` method to Service to indicate that the Characteristic belongs to this service
- compute the group_end_handles, characteristic handles, config_handles and value_handles.
- find a way to compute Service handle automatically too (when calling `set_gatt_table` ?)
- verify that there is no overlapping