Initializing SimGrid's model in another way

Initializing SimGrid's model in another way

Preparing the field to change models type to primary/virtual.
To do so, we need another way to set the "default" models in the root
netzone.

For that, we changed the order we created the zone during the XML
parser:
1) Create the netzone object (and set as root netzone if it's the first)
2) Create the simgrid's models (if
  2.1) During the initialization of simgrid's models (by the
surf_model_description_t structure), the default models are set in the
root netzone
3) If it's a child netzone, copy models from its father

Probably temporary changes:
- Create setters for each model in the NetZoneImpl. To be reviewed after
we decided where to put the "default" models.