Rationale: it was not really necessary (Algo.neighbor contains almost
the same information), and the Topology.neighbor type was alone in its
module, which looked weird.

Test: add enable rule so that step always do something (equivalent, but more in the spirit of the model)

Depending on the kind of network, we should raise an exception. Indeed
such information should not be available in anonymous networks.

We will add such checks later.

instead of duplicating its content

Fix: in the generated lutin demon, the constraint to prevent several actions to be activated in the same process was wrong

Actually, I've just realized that the I/O mode of rdbg already makes
it possible to use sasa from rdbg (exit event only). This plugin would
only be necessary when we would need a finer grained intrumentation, e.g.,
to stop at enable step, or to at each process call/exit.