Better default constructors, added default trees

src/main/scala/inox/InoxContext.scala

src/main/scala/inox/InoxOptions.scala

src/main/scala/inox/datagen/ModelEnumerator.scala

src/main/scala/inox/datagen/SolverDataGen.scala

src/main/scala/inox/evaluators/Evaluator.scala

src/main/scala/inox/evaluators/RecursiveEvaluator.scala

src/main/scala/inox/package.scala

src/main/scala/inox/solvers/GroundSolver.scala

src/main/scala/inox/solvers/NaiveAssumptionSolver.scala

src/main/scala/inox/solvers/Solver.scala

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