Function estimateJoinCardinality

Estimate the cardinality of a join. This estimation is done by:

1. Grouping operations together based on variables.
2. Selecting the minimum op cardinality in each group as the cardinality of that group.
3. Multiplying cardinalities of these (detached) groups.

This should provide a good balance between selective groups of operations, as well as cartesian joins between groups that do not overlap.