Add bi-directional adjacency logic

[brunorijsman]

The RIFT spec defines a bi-directional adjacency as follows:

Bi-directional Adjacency: Bidirectional adjacency is an adjacency
where nodes of both sides of the adjacency advertised it in the
node TIEs with the correct levels and system IDs. Bi-
directionality is used to check in different algorithms whether
the link should be included.

Add the following to RIFT-Python

• Logic to determine whether or not an adjacency is bi-directional
• Implement all the logic that depends on whether or not an adjacency is bi-directional (look in the spec to find out what this logic is)