Some ideas for general useful utilities

[To5BG]

1. intersection utility
   I think it would be nice to have a utility for tightening the domain of two variables based on their intersection, and then setting said domain to both of them. I can already see this being used for two of the propagators, for instance.

2. A way to access the variable that triggered propagation
   Simplifies the implementation of some propagators. For instance, this makes the circuit checking easier as you already have a starting point.