[ndnSIM] Add new FIB entry at forwarder

[Tetsuya Shigeyasu]

Hi Alex,

We would like to add new FIB entry at fowarder.cpp during simulation.

Currently, we are using GlobalRoutingHelper for default routing of any
incoming Interest.

For our new protocol, however, will switch Face for next-hop.
Hence, we are trying to implement to change FIB entry on the basis of NW  
condition as flexible.

For example, we would like to switch face1/face2 at router A.

consumer ---> router A (face 1) -----> router B ------> Producer
                   |                                |
                   |-(face 2) -------> router C ----


Would you please help us?

Best regards,
Tetsuya Shigeyasu