[WIP] ENH: add support for all types of problems in Bidirectional Search

[tirthasheshpatel]

Fixes #1144

I have added support for all types of problems in Bidirectional Search and their tests.

I am still confused about two things:

1. How to return the node containing the solution path along with the U value in BS? Maybe, we can also add that like all other search algorithms.
2. We need to traverse through each node to calculate the e (minimum edge value) value which can take exponential time for large problems. So, is it fine to initialize it to 0 or there is a way to calculate it?

[antmarakis]

1. Implementing this functionality like in the rest of the algorithms sounds like the way to go, definitely!

2. I think if we initialize them at 0 it would be fine. Maybe add a TODO comment next to the initialization to let people know maybe a better initialization can be explored.