P-problems should not have NP-complete problems show up as possible reductions to be added

[norkish]

For example, we can always reduce an NP-complete problem to a P problem (e.g., 3SAT to Deutsch's problem), but the opposite is not true (Deutsch's is not reducible to 3SAT). So we don't want people to see 3SAT in the dropdown menu in the Reduce pane when Deutsc's problem is selected in the Problem pane.

In short, we need to check that only problems B that CAN be reduced to from problems A show up as options in the Reduce panel when problem A is selected in the Panel pane.