Some work on preferential attachement generators.

Modified the old PreferentialAttachment to correctly use the
random generator of GeneratorBase (there was a bug if one changed
the random seed, it was not taken into account).

Also added a Barabàsi-Albert generator (BarabasiAlbertGenerator ...)
that does exaclty the same job but can produce graphs, not only trees.
Indeed the generator adds one node at each call to nextEvents(), for
this node it can add one edge (chosen randomly, with a bias toward
node with high degrees). However doing so creates a tree. One can
now make two ore more edges for each node added. The number of edges
generated at each node can be set. It can also be chosen randomly
(uniformly this time).

The BarabasiAlbertGenerator is stricly the same as the
PreferentialAttachmentGenerator when the number of edges added per
node is one. We can however keep the old generator for compatibility
reasons, or replace it by the BarabasiAlbertGenerator by changing
the name of one of the two.