NetZoneImpl: simpler constructor

NetZoneImpl: simpler constructor

NetZoneImpl(std::string& name);
One step towards a simple contructor and set() idiom.
Removed father and network model from constructor.

Collateral effect:

NetPoint:
- removed NetZoneImpl* from its constructor
- added method set_englobing_zone(NetZoneImpl*)

In the past, we could create the NetPoint in NetZoneImpl with the
pointer to the father. But now, the father is only set latter at
set_father method.