Moved model extraction to the right level in the encoding pipeline

src/main/scala/inox/solvers/SolverFactory.scala

src/main/scala/inox/solvers/unrolling/UnrollingSolver.scala

src/main/scala/inox/solvers/z3/AbstractZ3Solver.scala

src/main/scala/inox/solvers/z3/NativeZ3Solver.scala