Refactored WumpusKB and HybridWumpusAgent to use Expr obejcts (#862)

* Added ask_with_dpll to WumpusKB

* Refactored WumpusKB

* Refactored HybridWumpusAgent

* No need for ask_with_dpll, fix typos

* override ask_if_true in WumpusKB

* remove extra line