Add commands for forcing preferences during search

Add commands:

(prefer <formula>)
- will instruct case split queue to assign formula to true.
- prefer commands added within a scope are forgotten after leaving the scope.

(reset-preferences)
- resets asserted preferences. Has to be invoked at base level.

This provides functionality related to MathSAT and based on an ask by Tomáš Kolárik who is integrating the functionality with OpenSMT2

src/cmd_context/cmd_context.h

@@ -39,6 +39,7 @@ Notes:
 #include "solver/check_logic.h"
 #include "solver/progress_callback.h"
 #include "solver/simplifier_solver.h"
+#include "solver/preferred_value_propagator.h"
 #include "cmd_context/pdecl.h"
 #include "cmd_context/tactic_manager.h"
 #include "params/context_params.h"
@@ -265,6 +266,7 @@ protected:
     dictionary<object_ref*>      m_object_refs; // anything that can be named.
     dictionary<sexpr*>           m_user_tactic_decls;
     vector<std::pair<expr_ref, expr_ref>> m_var2values;
+    scoped_ptr<preferred_value_propagator> m_preferred;
 
     dictionary<func_decls>       m_func_decls;
     obj_map<func_decl, symbol>   m_func_decl2alias;
@@ -429,6 +431,8 @@ public:
     void set_solver(solver* s) { m_solver = s; }
     void set_proof_cmds(proof_cmds* pc) { m_proof_cmds = pc; }
     void set_initial_value(expr* var, expr* value);
+    void set_preferred(expr *fmla);
+    void reset_preferred();
 
     void set_solver_factory(solver_factory * s);
     void set_simplifier_factory(simplifier_factory& sf) { m_simplifier_factory = sf; }