Multigraph rules fixes: bialgebra

[RazinShaikh]

No description provided.

[RazinShaikh]

I have covered the three implementations of bialgebra in rules.py, basicrules.py and editor_actions.py. Let me know if that looks alright to you.

[jvdwetering]

I don't claim to fully understand what is going on here, but if you say that this indeed correctly implements the bialgebra rule and deals with all the weird edge cases you can have when you work with a multi-graph, then I'm happy to merge it.

[RazinShaikh]

I have tested with the weird cases and I believe it is correct. I can't guarantee that I haven't missed anything but previously the bialgebra rule was completely broken so this is at least an improvement.

Few comments:

1. The bialgebra in editor_actions.py is the one previously in basicrules.py modified to work with multiple edges and self-loops. Whenever there are no multi-edges, its behavior should match the one previously.
2. I have made the basicrules.py version use the bialgebra from editor_actions.py so there is less duplication.
3. The rules.py version of bialgebra is supposed to be used with automated rewrite strategies and it expects the number of vertices to always decrease. Whenever we have multiple edges between two nodes or self-loops, the number of vertices either stays the same or increases. So I have simply added a condition in the matcher that doesn't match any time there are multiple edges between those nodes or self-loops.