create Channel in opposite direction, if not exists

[sebulino]

It's possible that in the Channel Graph there's only information about one channel and information about its return channel is missing. Consequently they are currently not created in the UncertaintyNetwork (and possibly the OracleNetwork).

For the UncertaintyNetwork we still have enough information to create the return channel with a good enough estimate. Thus we should create them if needed. This could be done in the constructor or when the getter is called and would return none.