Breaking: remove Algo.pid and Algo.spid, and add the pid in the init function

The  rationale is  that the  Algo.spid that  gave processes  access to
their pid via their neighbors was a bit weird. Also, it makes the anonymity
of the algorithm more explicit: an access to the pid could appear anywhere
in the program, while now, the pid must be in state.t!

Now, if the  user needs the pid,  he must store it  explicitly in the
state via the initialization function.