Greatly simplify NotNode.dnf() by using Conjunct()/Disjunct().  Also, fix a bug where the dnf() call wasn't propogated when changing And to Or.
Greatly simplify NotNode.dnf() by using Conjunct()/Disjunct().  Also, fix a bug where the dnf() call wasn't propogated when changing And to Or.

eaburns/planit,eaburns/planit