Basically, the idea is to parse the result of the registred ('s ->
string) function.

For instance if  the user define 'v and 'v printer as follows:

 type state = { a : int ; b : float }
 let state_to_string s = Printf.sprintf "x=%d y=%f" s.v1 s.v2

pid_x and pid_y will be used as output var names in the RIF outputs.

If the user provide no name:
 let state_to_string s = Printf.sprintf "%d %f" s.v1 s.v2

some names will be invented (pid_v0 and pid_v1)

All the get_* functions (that are of no interest for users) are now in
the Register module.

Things that are currently broken:
- the ability to init vars in the dot
- controling the name of local vars in the rif output

The idea is to use the Obj module (arg !!!), which is safe thanks to
the fact that there is now only one register function, which forces all the
'v to be bound to the same type.

The rationale is that
 - it is not always the cmxs that is necessary .e.g., rdbg uses .cma or .ml
 - it makes more sense to refer to the source code

This can be used from rdbg from example, to generate session files.

In particular, 'rdbg -lurette' now works thanls to this option
(cf rdbg sha 63566a893807361691137cafe15854a2a7395eec)

Remove the --generate-rdbg-load one that is less general

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.

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.

This is necessary to be able to use sasa with luciole.  Indeed, in
rdbg/lurette, luciole always plays first. Hence if we want sasa to
play after luciole, we can use the option.

Note that from the lurette point of view, sasa plays the role of the env
as it needs to start playing, to provide Lutin the enable values for
the first demon step.

with algo several actions, and allow lurette to chose them.

and reuse it for Neighbor, which make the naming more consistent.

Also add the list of neighbors to the init function. The rationale is that
to give a sensible initial value to local vars of type 'Neighbor', we need
the neighbors number.

I could have chosen to pass an integer only. Well, who can the plus
can the minus.