Split-Duplex: new management

Split-Duplex: new management

1. Huge commit to add support to Split-Duplex in S4U.
  - New interface: s4u::SplitDuplexLink

2. Allows its creation through the C++ interface without manually creating
the link UP and DOWN.
  - NetZone::create_split_duplex_link

3. Changes in NetZone::add_route to allow the description of the
direction in C++ interface
  - LinkInRoute: wrap around Link* to indicate the direction
(UP/DOWN/NONE) for the link.
  - UP/DOWN are used to split-duplex links
  - NONE for other types (shared, wifi, fat-pipe)

4. Fix use of split-duplex links in symmetric routes in XML too.
     <route src="alice" dst="bob" symmetrical="YES">
       <link_ctn id="link1" direction="UP"/>
     </route>
  Now this code works, it will add the link1_DOWN as route to
bob->alice. In the past, it would add link1_UP for bob->alice too.