Refactoing temp vars, and reading vc solvers to use from command line

src/main/scala/leon/invariant/factories/AxiomFactory.scala

...scala/leon/invariant/factories/TemplateInstantiator.scala

src/main/scala/leon/invariant/structure/Formula.scala

...ala/leon/invariant/templateSolvers/NLTemplateSolver.scala

...ain/scala/leon/invariant/util/ExpressionTransformer.scala

...in/scala/leon/invariant/util/LetTupleSimplification.scala

src/main/scala/leon/invariant/util/SolverUtil.scala

src/main/scala/leon/invariant/util/TVarFactory.scala

src/main/scala/leon/laziness/LazinessEliminationPhase.scala

src/main/scala/leon/laziness/LazyVerificationPhase.scala

src/test/scala/leon/regression/orb/OrbRegressionSuite.scala