Enhance add_forwarding_rule for multipath routing

[MikaelAziz]

Modified add_forwarding_rule to support multiple next hops with probabilities for multipath routing. I am using this for my personal research project, but it should also be useful for various use cases and more complex network routing protocols. Additionally if this change is being adopted then a few of the tutorials should also be modified to include this.

[caitaozhan]

Hi Mikael,

Thanks. I see you changed the add_forwarding_rule, which results in failing some pytests. Please do the following:

1. Revert the change that happened to add_forwarding_rule
2. Add your new function add_forwarding_rule_multihop (or whatever name you want)
3. Update the pytest to make sure the unit test covers the newly added function.

Thanks,
Caitao