Triviality removal

CnfSatInstance now removes trivial assignments via the simplify() method
using the rules:

1) If a unit clause is found, assign the variable to satisfy the unit
clause
2) If a variable but not its negation appears, assign the variable to
true. Vice-versa
3) If there is a known assignment, remove all clauses containing
literals known to be true. Remove the negation of the variable from all
clauses.