This is necessary  to compute the heigth in a  tree and will probably
be useful for other purposes

Fix: the Register.diameter returned the links number (introduced in sha:40eece7c)

- add sub_tree_size, parent, is_in_tree, is_out_tree
- fix height (was only working for out-trees)
- a predessors to Topology.t
- refactor a lot of strange things in the code
  - in the use of hashtbl in particular

Doc: Add some odoc generated html files to the rdbg section of the user guide

- do not hard-code the size of the graph in the ml files
- do not print the whole state of big graphs. Indeed, for graphs of
  10000 nodes, if we print the path (which is itself an array of size 10000)
  we obtain 100 000 000 variables, which is too much for the rif file.

as it is where they belong.

Indeed, by using the Env.nset and Env.nget defined in
0d6575a7 a pb that was hided by the
use of Env.set and Env.get was triggered.

The bug was that variables initialisations were done several times.

In sasa.ml, the way I was updating the Env.t by iterating on the
variables was forcing to "close" expressions that were calling init
functions.

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.

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.

For some reason, I believed that the source dir containing the main
program need to be named bin for dune to work smoothly. But actually,
it seems to work fine with src, so let's go for it.